linux-kernel - Re: [PATCH v2] sched/wait: use freezable

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Date:   Thu, 7 Feb 2019 19:12:35 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Hugo Lefeuvre <hle@....eu.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: [PATCH v2] sched/wait: use freezable_schedule when possible

On Thu, Feb 07, 2019 at 09:03:52PM +0100, Hugo Lefeuvre wrote:
> Replace schedule(); try_to_freeze() by freezable_schedule().
> 
> Tasks calling freezable_schedule() set the PF_FREEZER_SKIP flag
> before calling schedule(). Unlike tasks calling schedule();
> try_to_freeze() tasks calling freezable_schedule() are not awaken by
> try_to_freeze_tasks(). Instead they call try_to_freeze() when they
> wake up if the freeze is still underway.
> 
> It is not a problem since sleeping tasks can't do anything which isn't
> allowed for a frozen task while sleeping.
> 
> The result is a potential performance gain during freeze, since less
> tasks have to be awaken.
> 
> For instance on a bare Debian vm running a 4.19 stable kernel, the
> number of tasks skipped in freeze_task() went up from 12 without the
> patch to 32 with the patch (out of 448), an increase of > x2.5.
> 

Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>


> Signed-off-by: Hugo Lefeuvre <hle@....eu.com>
> ---
> Changes in v2:
>   - Add test results to commit message.
>   - Split from initial patch set[0] since this patch can go in separately
>     [0] https://lkml.org/lkml/2019/2/1/19
> 
>  include/linux/wait.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/wait.h b/include/linux/wait.h
> index ed7c122cb31f..5f3efabc36f4 100644
> --- a/include/linux/wait.h
> +++ b/include/linux/wait.h
> @@ -308,7 +308,7 @@ do {										\
>  
>  #define __wait_event_freezable(wq_head, condition)				\
>  	___wait_event(wq_head, condition, TASK_INTERRUPTIBLE, 0, 0,		\
> -			    schedule(); try_to_freeze())
> +			    freezable_schedule())
>  
>  /**
>   * wait_event_freezable - sleep (or freeze) until a condition gets true
> @@ -367,7 +367,7 @@ do {										\
>  #define __wait_event_freezable_timeout(wq_head, condition, timeout)		\
>  	___wait_event(wq_head, ___wait_cond_timeout(condition),			\
>  		      TASK_INTERRUPTIBLE, 0, timeout,				\
> -		      __ret = schedule_timeout(__ret); try_to_freeze())
> +		      __ret = freezable_schedule_timeout(__ret))
>  
>  /*
>   * like wait_event_timeout() -- except it uses TASK_INTERRUPTIBLE to avoid
> @@ -588,7 +588,7 @@ do {										\
>  
>  #define __wait_event_freezable_exclusive(wq, condition)				\
>  	___wait_event(wq, condition, TASK_INTERRUPTIBLE, 1, 0,			\
> -			schedule(); try_to_freeze())
> +			freezable_schedule())
>  
>  #define wait_event_freezable_exclusive(wq, condition)				\
>  ({										\
> -- 
> 2.20.1