[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v2] x86/hpet: Improve handling of timer_deadline
>>> On 15.08.17 at 15:13, <andrew.cooper3@xxxxxxxxxx> wrote: > timer_deadline is only ever updated via this_cpu() in timer_softirq_action(), > so is not going to change behind the back of the currently running cpu. > > Update hpet_broadcast_{enter,exit}() to cache the value in a local variable to > avoid the repeated RELOC_HIDE() penalty. > > handle_hpet_broadcast() reads the timer_deadlines of remote cpus, but there is > no need to force the read for cpus which are not present in the mask. One > requirement is that we only sample the value once (which happens as a side > effect of RELOC_HIDE()), but is made more explicit with ACCESS_ONCE(). > > Bloat-o-meter shows a modest improvement: > > add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-144 (-144) > function old new delta > hpet_broadcast_exit 335 313 -22 > hpet_broadcast_enter 327 278 -49 > handle_hpet_broadcast 572 499 -73 > > Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx> with one nit: > @@ -714,9 +714,12 @@ void hpet_broadcast_enter(void) > cpumask_set_cpu(cpu, ch->cpumask); > > spin_lock(&ch->lock); > - /* reprogram if current cpu expire time is nearer */ > - if ( per_cpu(timer_deadline, cpu) < ch->next_event ) > - reprogram_hpet_evt_channel(ch, per_cpu(timer_deadline, cpu), NOW(), > 1); > + /* > + * reprogram if current cpu expire time is nearer. deadline is never > + * written by a remote cpu, so the value read earlier is still valid. > + */ Comments should start with an upper case letter. Jan _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx https://lists.xen.org/xen-devel
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