Messages in this thread | | | Subject | Re: [RFC PATCH] sched, timer: Use atomics for thread_group_cputimer stats | From | Jason Low <> | Date | Fri, 23 Jan 2015 15:39:21 -0800 |
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On Fri, 2015-01-23 at 21:08 +0100, Peter Zijlstra wrote: > On Fri, Jan 23, 2015 at 11:23:36AM -0800, Jason Low wrote: > > On Fri, 2015-01-23 at 10:25 +0100, Peter Zijlstra wrote: > > > On Thu, Jan 22, 2015 at 07:31:53PM -0800, Jason Low wrote: > > > > +static void update_gt_cputime(struct thread_group_cputimer *a, struct task_cputime *b) > > > > { > > > > + if (b->utime > atomic64_read(&a->utime)) > > > > + atomic64_set(&a->utime, b->utime); > > > > > > > > + if (b->stime > atomic64_read(&a->stime)) > > > > + atomic64_set(&a->stime, b->stime); > > > > > > > > + if (b->sum_exec_runtime > atomic64_read(&a->sum_exec_runtime)) > > > > + atomic64_set(&a->sum_exec_runtime, b->sum_exec_runtime); > > > > } > > > > > > See something like this is not safe against concurrent adds. > > > > How about something like: > > > > u64 a_utime, a_stime, a_sum_exec_runtime; > > > > retry_utime: > > a_utime = atomic64_read(&a->utime); > > if (b->utime > a_utime) { > > if (atomic64_cmpxchg(&a->utime, a_utime, b->utime) != a_utime) > > goto retry_utime; > > } > > > > retry_stime: > > a_stime = atomic64_read(&a->stime); > > if (b->stime > a_stime) { > > if (atomic64_cmpxchg(&a->stime, a_stime, b->stime) != a_stime) > > goto retry_stime; > > } > > > > retry_sum_exec_runtime: > > a_sum_exec_runtime = atomic64_read(&a->sum_exec_runtime); > > if (b->sum_exec_runtime > a_sum_exec_runtime) { > > if (atomic64_cmpxchg(&a->sum_exec_runtime, a_sum_exec_runtime, > > b->sum_exec_runtime) != a_sum_exec_runtime) > > goto retry_sum_exec_runtime; > > } > > Disgusting, at least use an inline or macro to avoid repeating it :-) > > Also, does anyone care about performance on 32bit systems? There's a few > where atomic64 is abysmal.
Yeah, though we're also avoiding spin lock/unlock calls each time, so not sure if we're really adding anything of significance to the "overall cost" on 32 bit systems. And update_gt_cputime wouldn't get called too frequently.
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