linux-kernel - Re: [PATCH v3] signals: Avoid unnecessary taking of sighand->siglock

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Date:   Tue, 18 Oct 2016 12:12:40 -0400
From:   Waiman Long <waiman.long@....com>
To:     Waiman Long <Waiman.Long@....com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stas Sergeev <stsp@...t.ru>, <linux-kernel@...r.kernel.org>,
        Scott J Norton <scott.norton@....com>,
        Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH v3] signals: Avoid unnecessary taking of sighand->siglock

On 09/27/2016 08:26 AM, Waiman Long wrote:
> When running certain database workload on a high-end system with many
> CPUs, it was found that spinlock contention in the sigprocmask syscalls
> became a significant portion of the overall CPU cycles as shown below.
>
>    9.30%  9.30%  905387  dataserver  /proc/kcore 0x7fff8163f4d2
>    [k] _raw_spin_lock_irq
>              |
>              ---_raw_spin_lock_irq
>                 |
>                 |--99.34%-- __set_current_blocked
>                 |          sigprocmask
>                 |          sys_rt_sigprocmask
>                 |          system_call_fastpath
>                 |          |
>                 |          |--50.63%-- __swapcontext
>                 |          |          |
>                 |          |          |--99.91%-- upsleepgeneric
>                 |          |
>                 |          |--49.36%-- __setcontext
>                 |          |          ktskRun
>
> Looking further into the swapcontext function in glibc, it was found
> that the function always call sigprocmask() without checking if there
> are changes in the signal mask.
>
> A check was added to the __set_current_blocked() function to avoid
> taking the sighand->siglock spinlock if there is no change in the
> signal mask. This will prevent unneeded spinlock contention when many
> threads are trying to call sigprocmask().
>
> With this patch applied, the spinlock contention in sigprocmask() was
> gone.
>
> Signed-off-by: Waiman Long<Waiman.Long@....com>
> ---
>   v2->v3:
>     - Add a sigequalsets() helper in signal.h and use it for comparison.
>
>   v1->v2:
>     - Fix compiler warning in mips.
>
>   include/linux/signal.h |   17 +++++++++++++++++
>   kernel/signal.c        |    7 +++++++
>   2 files changed, 24 insertions(+), 0 deletions(-)
>
> diff --git a/include/linux/signal.h b/include/linux/signal.h
> index b63f63e..5308304 100644
> --- a/include/linux/signal.h
> +++ b/include/linux/signal.h
> @@ -97,6 +97,23 @@ static inline int sigisemptyset(sigset_t *set)
>   	}
>   }
>
> +static inline int sigequalsets(const sigset_t *set1, const sigset_t *set2)
> +{
> +	switch (_NSIG_WORDS) {
> +	case 4:
> +		return	(set1->sig[3] == set2->sig[3])&&
> +			(set1->sig[2] == set2->sig[2])&&
> +			(set1->sig[1] == set2->sig[1])&&
> +			(set1->sig[0] == set2->sig[0]);
> +	case 2:
> +		return	(set1->sig[1] == set2->sig[1])&&
> +			(set1->sig[0] == set2->sig[0]);
> +	case 1:
> +		return	set1->sig[0] == set2->sig[0];
> +	}
> +	return 0;
> +}
> +
>   #define sigmask(sig)	(1UL<<  ((sig) - 1))
>
>   #ifndef __HAVE_ARCH_SIG_SETOPS
> diff --git a/kernel/signal.c b/kernel/signal.c
> index af21afc..04e8f50 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -2485,6 +2485,13 @@ void __set_current_blocked(const sigset_t *newset)
>   {
>   	struct task_struct *tsk = current;
>
> +	/*
> +	 * In case the signal mask hasn't changed, there is nothing we need
> +	 * to do. The current->blocked shouldn't be modified by other task.
> +	 */
> +	if (sigequalsets(&tsk->blocked, newset))
> +		return;
> +
>   	spin_lock_irq(&tsk->sighand->siglock);
>   	__set_task_blocked(tsk, newset);
>   	spin_unlock_irq(&tsk->sighand->siglock);

This is a pretty simple patch. Is that a chance that it can be pulled 
into 4.10?

Cheers,
Longman