Hello everyone, Before jumping into the issue, let me clarify the Cc list. Everyone have been cc'ed on Patch 0 through Patch 3. Respective arch maintainers, reviewers, and committers returned by scripts/get_maintainer.pl have been cc'ed on the respective arch side changes. Scheduler and CPU Idle maintainers and reviewers have been included for the entire series. If I have missed anyone, please do add them. If you would like to be dropped from the cc list, wholly or partially, for the future iterations, please do let me know. As long as the first three patches are applied in-order, the arch specific enablement can be applied independently and out-of-order since the TIF_NOTIFY_IPI flag is not used until Patch 3 and Patch 2 preps the complete tree to handle a break out of TIF_POLLING_NRFLAG state with the setting of either TIF_NOTIFY_IPI or TIF_NEED_RESCHED. Quick changelog and addressing concerns from v1 =============================================== v1: https://lore.kernel.org/lkml/20240220171457.703-1-kprateek.nayak@xxxxxxx/ v1..v2: o Rebased the series on latest tip:sched/core at commit c793a62823d1 ("sched/core: Drop spinlocks on contention iff kernel is preemptible") Fixed a conflict with commit edc8fc01f608 ("x86: Fix CPUIDLE_FLAG_IRQ_ENABLE leaking timer reprogram") that touched mwait_idle_with_hints() in arch/x86/include/asm/mwait.h o Dropping the ARM results since I never got my hands on the ARM64 system I used in my last testing. If I do manage to get my hands on it again, I'll rerun the experiments and share the results on the thread. To test the case where TIF_NOTIFY_IPI is not enabled for a particular architecture, I applied the series only until Patch 3 and tested the same on my x86 machine with a WARN_ON_ONCE() in do_idle() to check if tif_notify_ipi() ever return true and then repeated the same with Patch 4 applied. o Updated benchmark results based on the latest base. o Collected the Ack from Guo Ren for CSKY enablement. o Dropped the RFC tag. o Unfortunately, the series does not solve the issue highlighted by Julia Lawall w.r.t. NUMA Balancing in [0] based on her testing of v1. However, she did highlight a possible regression last time around where compiling a single file took much longer with the series but I could not reproduce it on my end. For sanity, I did rerun the same experiment this time around and I could not see any difference. Following are the numbers for $ make clean $ time make kernel/sched/core.o ---> tip:sched/core -j1 real 0m32.734s user 0m25.158s sys 0m6.750s -j256 real 0m7.181s user 0m27.509s sys 0m7.876s --> tip:sched/core + TIF_NOTIFY_IPI -j1 real 0m32.408s user 0m24.826s sys 0m6.767s -j256 real 0m7.187s user 0m27.556s sys 0m7.602s [0] https://lore.kernel.org/lkml/alpine.DEB.2.22.394.2310032059060.3220@hadrien/ Individual patches have their own changelog to help with review. With those details out of the way ... Problem statement ================= When measuring IPI throughput using a modified version of Anton Blanchard's ipistorm benchmark [1], configured to measure time taken to perform a fixed number of smp_call_function_single() (with wait set to 1), an increase in benchmark time was observed between v5.7 and the upstream release v6.7-rc6 (this was the latest upstream kernel at the time of encountering the issue). The issue persists on v6.10-rc1 as well. Bisection pointed to commit b2a02fc43a1f ("smp: Optimize send_call_function_single_ipi()") as the reason behind this increase in runtime. Experiments =========== Since the commit cannot be cleanly reverted on top of the current tip:sched/core, the effects of the optimizations were reverted by: 1. Removing the check for call_function_single_prep_ipi() in send_call_function_single_ipi(). With this change send_call_function_single_ipi() always calls arch_send_call_function_single_ipi() 2. Removing the call to flush_smp_call_function_queue() in do_idle() since every smp_call_function, with (1.), would unconditionally send an IPI to an idle CPU in TIF_POLLING mode. Following is the diff of the above described changes which will be henceforth referred to as the "revert": diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c index 31231925f1ec..735184d98c0f 100644 --- a/kernel/sched/idle.c +++ b/kernel/sched/idle.c @@ -332,11 +332,6 @@ static void do_idle(void) */ smp_mb__after_atomic(); - /* - * RCU relies on this call to be done outside of an RCU read-side - * critical section. - */ - flush_smp_call_function_queue(); schedule_idle(); if (unlikely(klp_patch_pending(current))) diff --git a/kernel/smp.c b/kernel/smp.c index f085ebcdf9e7..2ff100c41885 100644 --- a/kernel/smp.c +++ b/kernel/smp.c @@ -111,11 +111,9 @@ void __init call_function_init(void) static __always_inline void send_call_function_single_ipi(int cpu) { - if (call_function_single_prep_ipi(cpu)) { - trace_ipi_send_cpu(cpu, _RET_IP_, - generic_smp_call_function_single_interrupt); - arch_send_call_function_single_ipi(cpu); - } + trace_ipi_send_cpu(cpu, _RET_IP_, + generic_smp_call_function_single_interrupt); + arch_send_call_function_single_ipi(cpu); } static __always_inline void -- With the revert, the time taken to complete a fixed set of IPIs using ipistorm improves significantly. Following are the numbers from a dual socket 3rd Generation EPYC system (2 x 64C/128T) (boost on, C2 disabled) running ipistorm between CPU8 and CPU16: cmdline: insmod ipistorm.ko numipi=100000 single=1 offset=8 cpulist=8 wait=1 (tip:sched/core was at commit c793a62823d1 ("sched/core: Drop spinlocks on contention iff kernel is preemptible") for all the test data presented below) ================================================================== Test : ipistorm (modified) Units : Normalized runtime Interpretation: Lower is better Statistic : AMean ================================================================== kernel: time [pct imp] tip:sched/core 1.00 [00.00] tip:sched/core + revert 0.41 [60.00] Although the revert improves ipistorm performance, it also regresses tbench and netperf, supporting the validity of the optimization. Following are the tbench numbers from the same machine comparing vanilla tip:sched/core and the revert applied on top: ================================================================== Test : tbench Units : Normalized throughput Interpretation: Higher is better Statistic : AMean ================================================================== Clients: tip (CV) revert (CV) [pct imp] 1 1.00 (0.60) 0.90 (0.08) [-10%] 2 1.00 (0.27) 0.90 (0.76) [-10%] 4 1.00 (0.42) 0.90 (0.52) [-10%] 8 1.00 (0.78) 0.91 (0.54) [ -9%] 16 1.00 (1.70) 0.92 (0.39) [ -8%] 32 1.00 (1.73) 0.91 (1.39) [ -9%] 64 1.00 (1.09) 0.92 (1.60) [ -8%] 128 1.00 (1.45) 0.95 (0.52) [ -5%] 256 1.00 (0.96) 1.01 (0.28) [ 1%] 512 1.00 (0.32) 1.01 (0.20) [ 1%] 1024 1.00 (0.06) 1.01 (0.03) [ 1%] Since a simple revert is not a viable solution, we delved deeper into the changes in the execution path with call_function_single_prep_ipi() check. Effects of call_function_single_prep_ipi() ========================================== To pull a TIF_POLLING thread out of idle to process an IPI, the sender sets the TIF_NEED_RESCHED bit in the idle task's thread info in call_function_single_prep_ipi() and avoids sending an actual IPI to the target. As a result, the scheduler expects a task to be enqueued when exiting the idle path. This is not the case with non-polling idle states where the idle CPU exits the non-polling idle state to process the interrupt, and since need_resched() returns false, soon goes back to idle again. When TIF_NEED_RESCHED flag is set, do_idle() will call schedule_idle(), a large part of which runs with local IRQ disabled. In case of ipistorm, when measuring IPI throughput, this large IRQ disabled section delays processing of IPIs. Further auditing revealed that in absence of any runnable tasks, pick_next_task_fair(), which is called from the pick_next_task() fast path, will always call newidle_balance() in this scenario, further increasing the time spent in the IRQ disabled section. Following is the crude visualization of the problem with relevant functions expanded: -- CPU0 CPU1 ==== ==== do_idle() { __current_set_polling(); ... monitor(addr); if (!need_resched()) mwait() { /* Waiting */ smp_call_function_single(CPU1, func, wait = 1) { ... ... ... set_nr_if_polling(CPU1) { ... /* Realizes CPU1 is polling */ ... try_cmpxchg(addr, ... &val, ... val | _TIF_NEED_RESCHED); ... } /* Does not send an IPI */ ... ... } /* mwait exit due to write at addr */ csd_lock_wait() { } /* Waiting */ preempt_set_need_resched(); ... __current_clr_polling(); ... flush_smp_call_function_queue() { ... func(); } /* End of wait */ } } schedule_idle() { ... local_irq_disable(); smp_call_function_single(CPU1, func, wait = 1) { ... ... ... arch_send_call_function_single_ipi(CPU1); ... \ ... \ newidle_balance() { \ ... /* Delay */ ... \ } \ ... \--------------> local_irq_enable(); /* Processes the IPI */ -- Skipping newidle_balance() ========================== In an earlier attempt to solve the challenge of the long IRQ disabled section, newidle_balance() was skipped when a CPU waking up from idle was found to have no runnable tasks, and was transitioning back to idle [2]. Tim [3] and David [4] had pointed out that newidle_balance() may be viable for CPUs that are idling with tick enabled, where the newidle_balance() has the opportunity to pull tasks onto the idle CPU. Vincent [5] pointed out a case where the idle load kick will fail to run on an idle CPU since the IPI handler launching the ILB will check for need_resched(). In such cases, the idle CPU relies on newidle_balance() to pull tasks towards itself. Using an alternate flag instead of NEED_RESCHED to indicate a pending IPI was suggested as the correct approach to solve this problem on the same thread. Proposed solution: TIF_NOTIFY_IPI ================================= Instead of reusing TIF_NEED_RESCHED bit to pull an TIF_POLLING CPU out of idle, TIF_NOTIFY_IPI is a newly introduced flag that call_function_single_prep_ipi() sets on a target TIF_POLLING CPU to indicate a pending IPI, which the idle CPU promises to process soon. On architectures that do not support the TIF_NOTIFY_IPI flag (this series only adds support for x86 and ARM processors for now), call_function_single_prep_ipi() will fallback to setting TIF_NEED_RESCHED bit to pull the TIF_POLLING CPU out of idle. Since the pending IPI handlers are processed before the call to schedule_idle() in do_idle(), schedule_idle() will only be called if the IPI handler have woken / migrated a new task on the idle CPU and has set TIF_NEED_RESCHED bit to indicate the same. This avoids running into the long IRQ disabled section in schedule_idle() unnecessarily, and any need_resched() check within a call function will accurately notify if a task is waiting for CPU time on the CPU handling the IPI. Following is the crude visualization of how the situation changes with the newly introduced TIF_NOTIFY_IPI flag: -- CPU0 CPU1 ==== ==== do_idle() { __current_set_polling(); ... monitor(addr); if (!need_resched_or_ipi()) mwait() { /* Waiting */ smp_call_function_single(CPU1, func, wait = 1) { ... ... ... set_nr_if_polling(CPU1) { ... /* Realizes CPU1 is polling */ ... try_cmpxchg(addr, ... &val, ... val | _TIF_NOTIFY_IPI); ... } /* Does not send an IPI */ ... ... } /* mwait exit due to write at addr */ csd_lock_wait() { ... /* Waiting */ preempt_fold_need_resched(); /* fold if NEED_RESCHED */ ... __current_clr_polling(); ... flush_smp_call_function_queue() { ... func(); /* Will set NEED_RESCHED if sched_ttwu_pending() */ } /* End of wait */ } } if (need_resched()) { schedule_idle(); smp_call_function_single(CPU1, func, wait = 1) { } ... ... /* IRQs remain enabled */ arch_send_call_function_single_ipi(CPU1); -----------> /* Processes the IPI */ -- Results ======= With the TIF_NOTIFY_IPI, the time taken to complete a fixed set of IPIs using ipistorm improves drastically and is closer the numbers same with the revert. Following are the numbers from the same dual socket 3rd Generation EPYC system (2 x 64C/128T) (boost on, C2 disabled) running ipistorm between CPU8 and CPU16: cmdline: insmod ipistorm.ko numipi=100000 single=1 offset=8 cpulist=8 wait=1 ================================================================== Test : ipistorm (modified) Units : Normalized runtime Interpretation: Lower is better Statistic : AMean ================================================================== kernel: time [pct imp] tip:sched/core 1.00 [baseline] tip:sched/core + revert 0.40 [60.26] tip:sched/core + TIF_NOTIFY_IPI 0.46 [54.88] netperf and tbench results with the patch match the results on tip on the dual socket 3rd Generation AMD system (2 x 64C/128T). Additionally, hackbench, stream, and schbench too were tested, with results from the patched kernel matching that of the tip. Additional benefits =================== In nohz_csd_func(), the need_resched() check returns true when an idle CPU in TIF_POLLING mode is woken up to do an idle load balance which leads to the idle load balance bailing out early today since send_call_function_single_ipi() ends up setting the TIF_NEED_RESCHED flag to put the CPU out of idle and the flag is not cleared until __schedule() is called much later in the call path. With TIF_NOTIFY_IPI, this is no longer the case since TIF_NEED_RESCHED is only set if there is a genuine need to call schedule() and not used in an overloaded manner to notify a pending IPI. Links ===== [1] https://github.com/antonblanchard/ipistorm [2] https://lore.kernel.org/lkml/20240119084548.2788-1-kprateek.nayak@xxxxxxx/ [3] https://lore.kernel.org/lkml/b4f5ac150685456cf45a342e3bb1f28cdd557a53.camel@xxxxxxxxxxxxxxx/ [4] https://lore.kernel.org/lkml/20240123211756.GA221793@maniforge/ [5] https://lore.kernel.org/lkml/CAKfTPtC446Lo9CATPp7PExdkLhHQFoBuY-JMGC7agOHY4hs-Pw@xxxxxxxxxxxxxx/ This series is based on tip:sched/core at commit c793a62823d1 ("sched/core: Drop spinlocks on contention iff kernel is preemptible") -- Gautham R. Shenoy (4): thread_info: Add helpers to test and clear TIF_NOTIFY_IPI sched: Define a need_resched_or_ipi() helper and use it treewide sched/core: Use TIF_NOTIFY_IPI to notify an idle CPU in TIF_POLLING mode of pending IPI x86/thread_info: Introduce TIF_NOTIFY_IPI flag K Prateek Nayak (10): arm/thread_info: Introduce TIF_NOTIFY_IPI flag alpha/thread_info: Introduce TIF_NOTIFY_IPI flag openrisc/thread_info: Introduce TIF_NOTIFY_IPI flag powerpc/thread_info: Introduce TIF_NOTIFY_IPI flag sh/thread_info: Introduce TIF_NOTIFY_IPI flag sparc/thread_info: Introduce TIF_NOTIFY_IPI flag csky/thread_info: Introduce TIF_NOTIFY_IPI flag parisc/thread_info: Introduce TIF_NOTIFY_IPI flag nios2/thread_info: Introduce TIF_NOTIFY_IPI flag microblaze/thread_info: Introduce TIF_NOTIFY_IPI flag -- Cc: Richard Henderson <richard.henderson@xxxxxxxxxx> Cc: Ivan Kokshaysky <ink@xxxxxxxxxxxxxxxxxxxx> Cc: Matt Turner <mattst88@xxxxxxxxx> Cc: Russell King <linux@xxxxxxxxxxxxxxx> Cc: Guo Ren <guoren@xxxxxxxxxx> Cc: Michal Simek <monstr@xxxxxxxxx> Cc: Dinh Nguyen <dinguyen@xxxxxxxxxx> Cc: Jonas Bonn <jonas@xxxxxxxxxxxx> Cc: Stefan Kristiansson <stefan.kristiansson@xxxxxxxxxxxxx> Cc: Stafford Horne <shorne@xxxxxxxxx> Cc: "James E.J. Bottomley" <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> Cc: Helge Deller <deller@xxxxxx> Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx> Cc: Nicholas Piggin <npiggin@xxxxxxxxx> Cc: Christophe Leroy <christophe.leroy@xxxxxxxxxx> Cc: "Naveen N. Rao" <naveen.n.rao@xxxxxxxxxxxxx> Cc: Yoshinori Sato <ysato@xxxxxxxxxxxxxxxxxxxx> Cc: Rich Felker <dalias@xxxxxxxx> Cc: John Paul Adrian Glaubitz <glaubitz@xxxxxxxxxxxxxxxxxxx> Cc: "David S. Miller" <davem@xxxxxxxxxxxxx> Cc: Andreas Larsson <andreas@xxxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Borislav Petkov <bp@xxxxxxxxx> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx> Cc: "Rafael J. Wysocki" <rafael@xxxxxxxxxx> Cc: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Juri Lelli <juri.lelli@xxxxxxxxxx> Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx> Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx> Cc: Ben Segall <bsegall@xxxxxxxxxx> Cc: Mel Gorman <mgorman@xxxxxxx> Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx> Cc: Valentin Schneider <vschneid@xxxxxxxxxx> Cc: Andrew Donnellan <ajd@xxxxxxxxxxxxx> Cc: Benjamin Gray <bgray@xxxxxxxxxxxxx> Cc: Frederic Weisbecker <frederic@xxxxxxxxxx> Cc: Xin Li <xin3.li@xxxxxxxxx> Cc: Kees Cook <keescook@xxxxxxxxxxxx> Cc: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx> Cc: Tony Battersby <tonyb@xxxxxxxxxxxxxxx> Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx> Cc: Brian Gerst <brgerst@xxxxxxxxx> Cc: Leonardo Bras <leobras@xxxxxxxxxx> Cc: Imran Khan <imran.f.khan@xxxxxxxxxx> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx> Cc: Rik van Riel <riel@xxxxxxxxxxx> Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> Cc: David Vernet <void@xxxxxxxxxxxxx> Cc: Julia Lawall <julia.lawall@xxxxxxxx> Cc: linux-alpha@xxxxxxxxxxxxxxx Cc: linux-kernel@xxxxxxxxxxxxxxx Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx Cc: linux-csky@xxxxxxxxxxxxxxx Cc: linux-openrisc@xxxxxxxxxxxxxxx Cc: linux-parisc@xxxxxxxxxxxxxxx Cc: linuxppc-dev@xxxxxxxxxxxxxxxx Cc: linux-sh@xxxxxxxxxxxxxxx Cc: sparclinux@xxxxxxxxxxxxxxx Cc: linux-pm@xxxxxxxxxxxxxxx Cc: x86@xxxxxxxxxx -- arch/alpha/include/asm/thread_info.h | 2 ++ arch/arm/include/asm/thread_info.h | 3 ++ arch/csky/include/asm/thread_info.h | 2 ++ arch/microblaze/include/asm/thread_info.h | 2 ++ arch/nios2/include/asm/thread_info.h | 2 ++ arch/openrisc/include/asm/thread_info.h | 2 ++ arch/parisc/include/asm/thread_info.h | 2 ++ arch/powerpc/include/asm/thread_info.h | 2 ++ arch/sh/include/asm/thread_info.h | 2 ++ arch/sparc/include/asm/thread_info_32.h | 2 ++ arch/sparc/include/asm/thread_info_64.h | 2 ++ arch/x86/include/asm/mwait.h | 2 +- arch/x86/include/asm/thread_info.h | 2 ++ arch/x86/kernel/process.c | 2 +- drivers/cpuidle/cpuidle-powernv.c | 2 +- drivers/cpuidle/cpuidle-pseries.c | 2 +- drivers/cpuidle/poll_state.c | 2 +- include/linux/sched.h | 5 +++ include/linux/sched/idle.h | 12 +++---- include/linux/thread_info.h | 43 +++++++++++++++++++++++ kernel/sched/core.c | 41 ++++++++++++++++----- kernel/sched/idle.c | 23 ++++++++---- 22 files changed, 133 insertions(+), 26 deletions(-) -- 2.34.1
