From 43788e05b6583f018616c461603ae66b6f34b020 Mon Sep 17 00:00:00 2001 From: Mel Gorman Date: Sun, 24 May 2020 21:29:56 +0100 Subject: [PATCH] sched/core: Offload wakee task activation if it the wakee is descheduling The previous commit: c6e7bd7afaeb: ("sched/core: 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. The impact can be seen from a "perf sched latency" report generated from a single iteration of one packet size: ----------------------------------------------------------------------------------------------------------------- Task | Runtime ms | Switches | Average delay ms | Maximum delay ms | Maximum delay at | ----------------------------------------------------------------------------------------------------------------- vanilla netperf:4337 | 21709.193 ms | 2932 | avg: 0.002 ms | max: 0.041 ms | max at: 112.154512 s netserver:4338 | 14629.459 ms | 5146990 | avg: 0.001 ms | max: 1615.864 ms | max at: 140.134496 s localwakelist-v1r2 netperf:4339 | 29789.717 ms | 2460 | avg: 0.002 ms | max: 0.059 ms | max at: 138.205389 s netserver:4340 | 18858.767 ms | 7279005 | avg: 0.001 ms | max: 0.362 ms | max at: 135.709683 s ----------------------------------------------------------------------------------------------------------------- Note that the average wakeup delay is quite small on both the vanilla kernel and with the two patches applied. However, there are significant outliers with the vanilla kernel with the maximum one measured as 1615 milliseconds with a vanilla kernel but never worse than 0.362 ms with both patches applied and a much higher rate of context switching. Similarly a separate profile of cycles showed that 2.83% of all cycles were spent in try_to_wake_up() with almost half of the cycles spent on spinning on p->on_rq. With the two patches, the percentage of cycles spent in try_to_wake_up() drops to 1.13% Signed-off-by: Mel Gorman Signed-off-by: Ingo Molnar Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Jirka Hladky Cc: Vincent Guittot Cc: valentin.schneider@arm.com Cc: Hillf Danton Cc: Rik van Riel Link: https://lore.kernel.org/r/20200524202956.27665-3-mgorman@techsingularity.net Change-Id: I680c08132d1789995749a778f350bd77dee422dc Signed-off-by: Alexander Winkowski --- kernel/sched/core.c | 39 +++++++++++++++++++++++++++++++++------ kernel/sched/sched.h | 3 ++- 2 files changed, 35 insertions(+), 7 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 71e3c7f0860c..c82bccde804a 100755 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1890,7 +1890,13 @@ void scheduler_ipi(void) irq_exit(); } -static void __ttwu_queue_remote(struct task_struct *p, int cpu, int wake_flags) +/* + * Queue a task on the target CPUs wake_list and wake the CPU via IPI if + * necessary. The wakee CPU on receipt of the IPI will queue the task + * via sched_ttwu_wakeup() for activation so the wakee incurs the cost + * of the wakeup instead of the waker. + */ +static void __ttwu_queue_wakelist(struct task_struct *p, int cpu, int wake_flags) { struct rq *rq = cpu_rq(cpu); @@ -1933,11 +1939,32 @@ bool cpus_share_cache(int this_cpu, int that_cpu) return per_cpu(sd_llc_id, this_cpu) == per_cpu(sd_llc_id, that_cpu); } -static bool ttwu_queue_remote(struct task_struct *p, int cpu, int wake_flags) +static inline bool ttwu_queue_cond(int cpu, int wake_flags) +{ + /* + * If the CPU does not share cache, then queue the task on the + * remote rqs wakelist to avoid accessing remote data. + */ + if (!cpus_share_cache(smp_processor_id(), cpu)) + return true; + + /* + * If the task is descheduling and the only running task on the + * CPU then use the wakelist to offload the task activation to + * the soon-to-be-idle CPU as the current CPU is likely busy. + * nr_running is checked to avoid unnecessary task stacking. + */ + if ((wake_flags & WF_ON_RQ) && cpu_rq(cpu)->nr_running <= 1) + return true; + + return false; +} + +static bool ttwu_queue_wakelist(struct task_struct *p, int cpu, int wake_flags) { - if (sched_feat(TTWU_QUEUE) && !cpus_share_cache(smp_processor_id(), cpu)) { + if (sched_feat(TTWU_QUEUE) && ttwu_queue_cond(cpu, wake_flags)) { sched_clock_cpu(cpu); /* Sync clocks across CPUs */ - __ttwu_queue_remote(p, cpu, wake_flags); + __ttwu_queue_wakelist(p, cpu, wake_flags); return true; } @@ -1951,7 +1978,7 @@ static void ttwu_queue(struct task_struct *p, int cpu, int wake_flags) struct rq_flags rf; #if defined(CONFIG_SMP) - if (ttwu_queue_remote(p, cpu, wake_flags)) + if (ttwu_queue_wakelist(p, cpu, wake_flags)) return; #endif @@ -2208,7 +2235,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags, * let the waker make forward progress. This is safe because IRQs are * disabled and the IPI will deliver after on_cpu is cleared. */ - if (READ_ONCE(p->on_cpu) && ttwu_queue_remote(p, cpu, wake_flags)) + if (READ_ONCE(p->on_cpu) && ttwu_queue_wakelist(p, cpu, wake_flags | WF_ON_RQ)) goto unlock; /* diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 051f6700d293..548893bebbfd 100755 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -1596,7 +1596,8 @@ static inline void finish_lock_switch(struct rq *rq, struct task_struct *prev) */ #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 */ /* * To aid in avoiding the subversion of "niceness" due to uneven distribution