linux-kernel - Re: [PATCH 3/3] timer: Reduce unnecessary sighand lock contention

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Message-ID: <20150827211219.GA25501@lerouge>
Date:	Thu, 27 Aug 2015 23:12:21 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Jason Low <jason.low2@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Terry Rudd <terry.rudd@...com>, Rik van Riel <riel@...hat.com>,
	Scott J Norton <scott.norton@...com>
Subject: Re: [PATCH 3/3] timer: Reduce unnecessary sighand lock contention

On Thu, Aug 27, 2015 at 01:29:50PM -0700, Jason Low wrote:
> On Thu, 2015-08-27 at 14:53 +0200, Frederic Weisbecker wrote:
> > Sure, like:
> > 
> > #define CPUTIMER_RUNNING 0x1
> > #define CPUTIMER_CHECKING 0x2
> > 
> > struct thread_group_cputimer {
> >     struct task_cputime_atomic cputime_atomic;
> >     int status;
> > }
> > 
> > So from cputimer_running() you just need to check:
> > 
> >      if (cputimer->status & CPUTIMER_RUNNING)
> > 
> > And from run_posix_cpu_timer() fast-path:
> > 
> >      if (cputimer->status == CPUTIMER_RUNNING)
> > 
> > so that ignores CPUTIMER_CHECKING case.
> 
> Right, having just 1 "status" field can simply things a bit. The
> (cputimer->status == CPUTIMER_RUNNING) check does appear misleading
> though, since we're technically not only checking for if the "cputimer
> is running".
> 
> Maybe something like:
> 
> int status = cputimer->status;
> if ((status & CPUTIMER_RUNNING) && !(status & CPUTIMER_CHECKING))
> 
> makes it more obvious what's going on here.

The result is the same but it may clarify the code indeed.

Thanks.
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