On Sun, May 24, 2020 at 09:29:56PM +0100, Mel Gorman wrote: > The patch "sched: Optimize ttwu() spinning on p->on_cpu" avoids spinning > on p->on_rq when the task is descheduling but only if the wakee is on > a CPU that does not share cache with the waker. This patch offloads the > activation of the wakee to the CPU that is about to go idle if the task > is the only one on the runqueue. This potentially allows the waker task > to continue making progress when the wakeup is not strictly synchronous. > > This is very obvious with netperf UDP_STREAM running on localhost. The > waker is sending packets as quickly as possible without waiting for any > reply. It frequently wakes the server for the processing of packets and > when netserver is using local memory, it quickly completes the processing > and goes back to idle. The waker often observes that netserver is on_rq > and spins excessively leading to a drop in throughput. > > This is a comparison of 5.7-rc6 against "sched: Optimize ttwu() spinning > on p->on_cpu" and against this patch labeled vanilla, optttwu-v1r1 and > localwakelist-v1r2 respectively. > > 5.7.0-rc6 5.7.0-rc6 > 5.7.0-rc6 > vanilla optttwu-v1r1 > localwakelist-v1r2 > Hmean send-64 251.49 ( 0.00%) 258.05 * 2.61%* > 305.59 * 21.51%* > Hmean send-128 497.86 ( 0.00%) 519.89 * 4.43%* > 600.25 * 20.57%* > Hmean send-256 944.90 ( 0.00%) 997.45 * 5.56%* > 1140.19 * 20.67%* > Hmean send-1024 3779.03 ( 0.00%) 3859.18 * 2.12%* > 4518.19 * 19.56%* > Hmean send-2048 7030.81 ( 0.00%) 7315.99 * 4.06%* > 8683.01 * 23.50%* > Hmean send-3312 10847.44 ( 0.00%) 11149.43 * 2.78%* > 12896.71 * 18.89%* > Hmean send-4096 13436.19 ( 0.00%) 13614.09 ( 1.32%) > 15041.09 * 11.94%* > Hmean send-8192 22624.49 ( 0.00%) 23265.32 * 2.83%* > 24534.96 * 8.44%* > Hmean send-16384 34441.87 ( 0.00%) 36457.15 * 5.85%* > 35986.21 * 4.48%* > > Note that this benefit is not universal to all wakeups, it only applies > to the case where the waker often spins on p->on_rq. > Thanks for the detailed description. I think you meant p->on_cpu here not p->on_rq. I know this patch is included, if possible, please make this edit here and below.
> /* > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > index db3a57675ccf..06297d1142a0 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -1688,7 +1688,8 @@ static inline int task_on_rq_migrating(struct > task_struct *p) > */ > #define WF_SYNC 0x01 /* Waker goes to sleep > after wakeup */ > #define WF_FORK 0x02 /* Child wakeup after > fork */ > -#define WF_MIGRATED 0x4 /* Internal use, task got > migrated */ > +#define WF_MIGRATED 0x04 /* Internal use, task got > migrated */ > +#define WF_ON_RQ 0x08 /* Wakee is on_rq */ > should this be WF_ON_CPU? There is a different check for p->on_rq in try_to_wake_up. Thanks, Pavan -- Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc. Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
