On Mon, 2015-03-02 at 11:03 -0800, Linus Torvalds wrote: > On Mon, Mar 2, 2015 at 10:42 AM, Jason Low <jason.l...@hp.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.
Okay, I will run some tests to see how this change affects the performance of itimers on 32 bit systems. While the update_gt_cputime() shouldn't be an issue for performance since it doesn't get called often, the sample_group_cputimer() needing to take locks 3 times for each atomic64_read is something that could impact performance, so we should take a look at that. Thanks, Jason -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
