Message-ID: <5540ECDA.6060308@redhat.com>
Date: Wed, 29 Apr 2015 10:38:18 -0400
From: Rik van Riel <riel@redhat.com>
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To: Jason Low <jason.low2@hp.com>, Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>
CC: linux-kernel@vger.kernel.org,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>, Mel Gorman <mgorman@suse.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Preeti U Murthy <preeti@linux.vnet.ibm.com>,
        Mike Galbraith <umgwanakikbuti@gmail.com>,
        Davidlohr Bueso <dave@stgolabs.net>, Waiman Long <Waiman.Long@hp.com>,
        Aswin Chandramouleeswaran <aswin@hp.com>,
        Scott J Norton <scott.norton@hp.com>
Subject: Re: [PATCH v2 3/5] sched, timer: Use atomics in thread_group_cputimer
 to improve scalability
References: <1430251224-5764-1-git-send-email-jason.low2@hp.com> <1430251224-5764-4-git-send-email-jason.low2@hp.com>
In-Reply-To: <1430251224-5764-4-git-send-email-jason.low2@hp.com>
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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@hp.com>

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

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