The following commit has been merged into the x86/entry branch of tip: Commit-ID: 8fbf48a5cdb83a1ae4285920713facef72639641 Gitweb: https://git.kernel.org/tip/8fbf48a5cdb83a1ae4285920713facef72639641 Author: Thomas Gleixner <tglx@xxxxxxxxxxxxx> AuthorDate: Thu, 21 May 2020 22:05:17 +02:00 Committer: Ingo Molnar <mingo@xxxxxxxxxx> CommitterDate: Tue, 26 May 2020 19:06:27 +02:00 x86/entry: Provide idtentry_entry/exit_cond_rcu() After a lengthy discussion [1] it turned out that RCU does not need a full rcu_irq_enter/exit() when RCU is already watching. All it needs if NOHZ_FULL is active is to check whether the tick needs to be restarted. This allows to avoid a separate variant for the pagefault handler which cannot invoke rcu_irq_enter() on a kernel pagefault which might sleep. The cond_rcu argument is only temporary and will be removed once the existing users of idtentry_enter/exit() have been cleaned up. After that the code can be significantly simplified. [ mingo: Simplified the control flow ] Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> Acked-by: "Paul E. McKenney" <paulmck@xxxxxxxxxx> Acked-by: Andy Lutomirski <luto@xxxxxxxxxx> Link: [1] https://lkml.kernel.org/r/20200515235125.628629605@xxxxxxxxxxxxx Link: https://lore.kernel.org/r/20200521202117.181397835@xxxxxxxxxxxxx --- arch/x86/entry/common.c | 79 +++++++++++++++++++++++++------- arch/x86/include/asm/idtentry.h | 14 +++++- 2 files changed, 76 insertions(+), 17 deletions(-) diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c index 9ebe334..a7f5846 100644 --- a/arch/x86/entry/common.c +++ b/arch/x86/entry/common.c @@ -512,8 +512,10 @@ SYSCALL_DEFINE0(ni_syscall) } /** - * idtentry_enter - Handle state tracking on idtentry + * idtentry_enter_cond_rcu - Handle state tracking on idtentry with conditional + * RCU handling * @regs: Pointer to pt_regs of interrupted context + * @cond_rcu: Invoke rcu_irq_enter() only if RCU is not watching * * Invokes: * - lockdep irqflag state tracking as low level ASM entry disabled @@ -521,40 +523,84 @@ SYSCALL_DEFINE0(ni_syscall) * * - Context tracking if the exception hit user mode. * - * - RCU notification if the exception hit kernel mode. - * * - The hardirq tracer to keep the state consistent as low level ASM * entry disabled interrupts. + * + * For kernel mode entries RCU handling is done conditional. If RCU is + * watching then the only RCU requirement is to check whether the tick has + * to be restarted. If RCU is not watching then rcu_irq_enter() has to be + * invoked on entry and rcu_irq_exit() on exit. + * + * Avoiding the rcu_irq_enter/exit() calls is an optimization but also + * solves the problem of kernel mode pagefaults which can schedule, which + * is not possible after invoking rcu_irq_enter() without undoing it. + * + * For user mode entries enter_from_user_mode() must be invoked to + * establish the proper context for NOHZ_FULL. Otherwise scheduling on exit + * would not be possible. + * + * Returns: True if RCU has been adjusted on a kernel entry + * False otherwise + * + * The return value must be fed into the rcu_exit argument of + * idtentry_exit_cond_rcu(). */ -void noinstr idtentry_enter(struct pt_regs *regs) +bool noinstr idtentry_enter_cond_rcu(struct pt_regs *regs, bool cond_rcu) { if (user_mode(regs)) { enter_from_user_mode(); - } else { + return false; + } + + if (!cond_rcu || !__rcu_is_watching()) { + /* + * If RCU is not watching then the same careful + * sequence vs. lockdep and tracing is required + * as in enter_from_user_mode(). + * + * This only happens for IRQs that hit the idle + * loop, i.e. if idle is not using MWAIT. + */ lockdep_hardirqs_off(CALLER_ADDR0); rcu_irq_enter(); instrumentation_begin(); trace_hardirqs_off_prepare(); instrumentation_end(); + + return true; } + + /* + * If RCU is watching then RCU only wants to check + * whether it needs to restart the tick in NOHZ + * mode. + */ + instrumentation_begin(); + rcu_irq_enter_check_tick(); + /* Use the combo lockdep/tracing function */ + trace_hardirqs_off(); + instrumentation_end(); + + return false; } /** - * idtentry_exit - Common code to handle return from exceptions + * idtentry_exit_cond_rcu - Handle return from exception with conditional RCU + * handling * @regs: Pointer to pt_regs (exception entry regs) + * @rcu_exit: Invoke rcu_irq_exit() if true * * Depending on the return target (kernel/user) this runs the necessary - * preemption and work checks if possible and required and returns to + * preemption and work checks if possible and reguired and returns to * the caller with interrupts disabled and no further work pending. * * This is the last action before returning to the low level ASM code which * just needs to return to the appropriate context. * - * Invoked by all exception/interrupt IDTENTRY handlers which are not - * returning through the paranoid exit path (all except NMI, #DF and the IST - * variants of #MC and #DB) and are therefore on the thread stack. + * Counterpart to idtentry_enter_cond_rcu(). The return value of the entry + * function must be fed into the @rcu_exit argument. */ -void noinstr idtentry_exit(struct pt_regs *regs) +void noinstr idtentry_exit_cond_rcu(struct pt_regs *regs, bool rcu_exit) { lockdep_assert_irqs_disabled(); @@ -580,7 +626,8 @@ void noinstr idtentry_exit(struct pt_regs *regs) if (IS_ENABLED(CONFIG_DEBUG_ENTRY)) WARN_ON_ONCE(!on_thread_stack()); instrumentation_begin(); - rcu_irq_exit_preempt(); + if (rcu_exit) + rcu_irq_exit_preempt(); if (need_resched()) preempt_schedule_irq(); /* Covers both tracing and lockdep */ @@ -602,10 +649,12 @@ void noinstr idtentry_exit(struct pt_regs *regs) trace_hardirqs_on_prepare(); lockdep_hardirqs_on_prepare(CALLER_ADDR0); instrumentation_end(); - rcu_irq_exit(); + if (rcu_exit) + rcu_irq_exit(); lockdep_hardirqs_on(CALLER_ADDR0); } else { - /* IRQ flags state is correct already. Just tell RCU */ - rcu_irq_exit(); + /* IRQ flags state is correct already. Just tell RCU. */ + if (rcu_exit) + rcu_irq_exit(); } } diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h index ce97478..a116b80 100644 --- a/arch/x86/include/asm/idtentry.h +++ b/arch/x86/include/asm/idtentry.h @@ -7,8 +7,18 @@ #ifndef __ASSEMBLY__ -void idtentry_enter(struct pt_regs *regs); -void idtentry_exit(struct pt_regs *regs); +bool idtentry_enter_cond_rcu(struct pt_regs *regs, bool cond_rcu); +void idtentry_exit_cond_rcu(struct pt_regs *regs, bool rcu_exit); + +static __always_inline void idtentry_enter(struct pt_regs *regs) +{ + idtentry_enter_cond_rcu(regs, false); +} + +static __always_inline void idtentry_exit(struct pt_regs *regs) +{ + idtentry_exit_cond_rcu(regs, true); +} /** * DECLARE_IDTENTRY - Declare functions for simple IDT entry points
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