On Wed, Dec 16, 2020 at 08:59:30AM -0800, Paul E. McKenney wrote: > On Fri, Nov 13, 2020 at 01:13:15PM +0100, Frederic Weisbecker wrote: > > > > Frederic Weisbecker (19): > > rcu/nocb: Turn enabled/offload states into a common flag > > rcu/nocb: Provide basic callback offloading state machine bits > > rcu/nocb: Always init segcblist on CPU up > > rcu/nocb: De-offloading CB kthread > > rcu/nocb: Don't deoffload an offline CPU with pending work > > rcu/nocb: De-offloading GP kthread > > rcu/nocb: Re-offload support > > rcu/nocb: Shutdown nocb timer on de-offloading > > rcu: Flush bypass before setting SEGCBLIST_SOFTIRQ_ONLY > > rcu/nocb: Set SEGCBLIST_SOFTIRQ_ONLY at the very last stage of de-offloading > > rcu/nocb: Only cond_resched() from actual offloaded batch processing > > rcu/nocb: Process batch locally as long as offloading isn't complete > > rcu/nocb: Locally accelerate callbacks as long as offloading isn't complete > > tools/rcutorture: Support nocb toggle in TREE01 > > I applied the above, with the usual commit-log wordsmithing. > > > rcutorture: Remove weak nocb declarations > > rcutorture: Export nocb (de)offloading functions > > These I folded into the rcutorture commit, as you suggested. Good! > > > cpu/hotplug: Add lockdep_is_cpus_held() > > timer: Add timer_curr_running() > > I applied these two. > > > rcu/nocb: Detect unsafe checks for offloaded rdp > > This one didn't apply, probably due to recent changes in -rcu. Could > you please take a look? Ok I'm going to rebase it. > I believe that the following enhancements will be needed: > > o Forbid toggling of offlined CPUs. This is a simple rule that > results in deterministic success or failure. The current setup > (which I freely admit that I suggested) will fail very rarely, > only if a newly offlined offloaded CPU is toggled before its > remaining callbacks are invoked. The current state is therefore > an accident waiting to happen. > > This will entail fixing a few comments as well. Sure, I'm always glad to remove lines! > > o Drain the bypass early in the de-offloading process and prohibit > queuing onto the bypass anywhere in the toggling process. > Then the only CPUs permitted to use the bypass are those that > are fully offloaded. This approach allows rcu_core() and the > rcuog kthreads to safely manipulate callbacks and grace periods > concurrently. (Famous last words!) This might address the > rcutorture writer stall warnings I am seeing. > > This will entail fixing a few comments as well. Ok, I'm checking that. > > o Avoid double write to rdp->nocb_cb_sleep in nocb_cb_wait(). > Unless there is some reason why this is absolutely required. > It will cause confusion as it is. Ok. > > o What bad thing happens if we de-offload the CPU corresponding to > the nocb GP kthread? It does work fine when that CPU is offlined. > > Coincidence or not, all but one of the rcutorture writer stalls > is followed by a message refusing to de-offload this CPU. It probably started with a brainfart that I elaborated a bit too far. Let me check that again. > > o The nocb_gp_update_state() function's return value seems backwards. > What am I missing here? Indeed I should rename it to nocb_gp_check_rdp() or something. > > o __rcu_nocb_rdp_deoffload(): Why move rcu_nocb_lock_irqsave() > across a comment? Because the comment only concerns the locking? > > o We need better debug. For example, "Can't deoffload an rdp GP > leader (yet)". Well, which CPU? For another example, we need > deoffload state in rcutorture writer-stall dumps. Ok. > > I intend to do the following, and, if feasible, fold them into the > original commits: > > o Make rcu_segcblist_is_offloaded() use "&&" instead of a > nested "if" statement. > > o nocb_cb_wait() needs the "^^^ Ensure CB invocation follows > _sleep test" to be adjusted so that it is properly attached to > its smp_load_acquire(). > > o nocb_cb_wait() -- alphabetical order for local variables, please! > Yeah, I know, others like inverted tree for reasons that escape me > completely. Ah, ok noted. Thanks! > Thanx, Paul > > > include/linux/cpu.h | 1 + > > include/linux/rcu_segcblist.h | 119 +++++- > > include/linux/rcupdate.h | 4 + > > include/linux/timer.h | 2 + > > kernel/cpu.c | 7 + > > kernel/rcu/rcu_segcblist.c | 13 +- > > kernel/rcu/rcu_segcblist.h | 45 ++- > > kernel/rcu/rcutorture.c | 3 - > > kernel/rcu/tree.c | 49 ++- > > kernel/rcu/tree.h | 2 + > > kernel/rcu/tree_plugin.h | 416 +++++++++++++++++++-- > > kernel/time/timer.c | 13 + > > .../selftests/rcutorture/configs/rcu/TREE01.boot | 4 +- > > 13 files changed, 614 insertions(+), 64 deletions(-)
