linux-kernel - Re: [PATCH v2] sched, timer: Use atomics for thread_group

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Date:	Mon, 2 Mar 2015 11:03:55 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jason Low <jason.low2@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Scott Norton <scott.norton@...com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] sched, timer: Use atomics for thread_group_cputimer to
 improve scalability

On Mon, Mar 2, 2015 at 10:42 AM, Jason Low <jason.low2@...com> wrote:
>
> 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%.

NAK.

Not because I think this is wrong, but because somebody needs to look
at the effects on 32-bit architectures too.

In particular, check out lib/atomic64.c - which uses a hashed array of
16-bit spinlocks to do 64-bit atomics. That may or may well work ok in
practice, but it does mean that now sample_group_cputimer() and
update_gt_cputime() will take that (it ends up generally being the
same) spinlock three times for the three atomic64_read()'s.

Now, I think on x86, we end up using not lib/atomic64.c but our own
versions that use cmpxchg8b, which is probably fine from a performance
standpoint. But I see a lot of "select GENERIC_ATOMIC64" for other
architectures.

Anyway, it is *possible* that even on those 32-bit targets, the
atomic64's aren't any worse than the current spinlock in practice.  So
the "NAK" is in no way absolute - but I'd just like to hear that this
is all reasonably fine on 32-bit ARM and powerpc, for example.

Hmm?

                             Linus
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