linux-kernel - Re: [PATCH v2 3/5] sched, timer: Use atomics in thread_group

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Message-ID: <5540ECDA.6060308@redhat.com>
Date:	Wed, 29 Apr 2015 10:38:18 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Jason Low <jason.low2@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
CC:	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mel Gorman <mgorman@...e.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Waiman Long <Waiman.Long@...com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Scott J Norton <scott.norton@...com>
Subject: Re: [PATCH v2 3/5] sched, timer: Use atomics in thread_group_cputimer
 to improve scalability

On 04/28/2015 04:00 PM, Jason Low wrote:
> While running a database workload, we found a scalability issue with itimers.
>
> Much of the problem was caused by the thread_group_cputimer spinlock.
> Each time we account for group system/user time, we need to obtain a
> thread_group_cputimer's spinlock to update the timers. On larger systems
> (such as a 16 socket machine), this caused more than 30% of total time
> spent trying to obtain this kernel lock to update these group timer stats.
>
> This patch converts the timers to 64 bit atomic variables and use
> atomic add to update them without a lock. With this patch, the percent
> of total time spent updating thread group cputimer timers was reduced
> from 30% down to less than 1%.
>
> Note: On 32 bit systems using the generic 64 bit atomics, this causes
> sample_group_cputimer() to take locks 3 times instead of just 1 time.
> However, we tested this patch on a 32 bit system ARM system using the
> generic atomics and did not find the overhead to be much of an issue.
> An explanation for why this isn't an issue is that 32 bit systems usually
> have small numbers of CPUs, and cacheline contention from extra spinlocks
> called periodically is not really apparent on smaller systems.

I don't see 32 bit systems ever getting so many CPUs
that this becomes an issue :)

> Signed-off-by: Jason Low <jason.low2@...com>

Acked-by: Rik van Riel <riel@...hat.com>

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