linux-kernel - Re: [PATCH v3 tip/core/timers 0/5] Crude timer-wheel latency hacks

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Message-ID: <20140116094420.41d435c5@gandalf.local.home>
Date:	Thu, 16 Jan 2014 09:44:20 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, fweisbec@...il.com, oleg@...hat.com, sbw@....edu
Subject: Re: [PATCH v3 tip/core/timers 0/5] Crude timer-wheel latency hacks

On Wed, 15 Jan 2014 20:02:03 -0800
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> Hello!
> 
> The following five patches provide some crude timer-wheel latency patches.
> I understand that a more comprehensive solution is in progress, but in the
> meantime, these patches work well in cases where a given CPU has either
> zero or one timers pending, which is a common case for NO_HZ_FULL kernels.
> Note that these patches do not help in the case where a given timer wheel
> has a pair of widely separated timers, while the more comprehensive
> solution is likely to handle more gracefully.  So, on the off-chance
> that this is helpful to someone, the individual patches are as follows:
> 
> 1.	Add ->all_timers field to tbase_vec to count all timers, not
> 	just the non-deferrable ones.
> 
> 2.	Avoid jiffy-at-a-time stepping when the timer wheel is empty.
> 
> 3.	Avoid jiffy-at-a-time stepping when the timer wheel transitions
> 	to empty.
> 
> 4.	Avoid jiffy-at-a-time stepping after a timer is added to an
> 	initially empty timer wheel.
> 
> 5.	Make internal_add_timer() update ->next_timer if ->active_timers == 0,
> 	courtesy of Oleg Nesterov.
> 

Although I think the "(void)" is ugly and uncalled for (there's lots of
functions that return a value in the kernel that don't have that). But,
it's not up to me to argue this.

For all 5 patches,

Reviewed-by: Steven Rostedt <rostedt@...dmis.org>

-- Steve
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