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SubjectRe: [patch V9 04/39] x86/entry: Provide idtentry_entry/exit_cond_rcu()

* Thomas Gleixner <tglx@linutronix.de> wrote:

> From: Thomas Gleixner <tglx@linutronix.de>
>
> 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.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Link: [1] https://lkml.kernel.org/r/20200515235125.628629605@linutronix.de
> ---
> V9: Reworked to the new RCU mode
> ---
> arch/x86/entry/common.c | 84 ++++++++++++++++++++++++++++++----------
> arch/x86/include/asm/idtentry.h | 14 +++++-
> 2 files changed, 77 insertions(+), 21 deletions(-)
>
> --- 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 +523,81 @@ 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 {
> - lockdep_hardirqs_off(CALLER_ADDR0);
> - rcu_irq_enter();
> - instrumentation_begin();
> - trace_hardirqs_off_prepare();
> - instrumentation_end();
> + 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 hits 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;
> + } else {
> + /*
> + * 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;
> }

Any objections to the simplified/flattened control flow below?

Thanks,

Ingo

common.c | 59 +++++++++++++++++++++++++++++++----------------------------
1 file changed, 31 insertions(+), 28 deletions(-)
Index: tip/arch/x86/entry/common.c
===================================================================
--- tip.orig/arch/x86/entry/common.c
+++ tip/arch/x86/entry/common.c
@@ -549,35 +549,38 @@ bool noinstr idtentry_enter_cond_rcu(str
{
if (user_mode(regs)) {
enter_from_user_mode();
- } else {
- 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;
- } else {
- /*
- * 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;
}
+
+ 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;
}

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