Re: [PATCH v2] sched, timer: Use atomics for thread_group_cputimer to improve scalability

[Linus Torvalds]

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