Messages in this thread | | | From | Vincent Guittot <> | Date | Thu, 19 Oct 2023 14:08:47 +0200 | Subject | Re: [PATCH v11 3/3] sched/fair: Use candidate prev/recent_used CPU if scanning failed for cluster wakeup |
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On Thu, 19 Oct 2023 at 05:36, Yicong Yang <yangyicong@huawei.com> wrote: > > From: Yicong Yang <yangyicong@hisilicon.com> > > Chen Yu reports a hackbench regression of cluster wakeup when > hackbench threads equal to the CPU number [1]. Analysis shows > it's because we wake up more on the target CPU even if the > prev_cpu is a good wakeup candidate and leads to the decrease > of the CPU utilization. > > Generally if the task's prev_cpu is idle we'll wake up the task > on it without scanning. On cluster machines we'll try to wake up > the task in the same cluster of the target for better cache > affinity, so if the prev_cpu is idle but not sharing the same > cluster with the target we'll still try to find an idle CPU within > the cluster. This will improve the performance at low loads on > cluster machines. But in the issue above, if the prev_cpu is idle > but not in the cluster with the target CPU, we'll try to scan an > idle one in the cluster. But since the system is busy, we're > likely to fail the scanning and use target instead, even if > the prev_cpu is idle. Then leads to the regression. > > This patch solves this in 2 steps: > o record the prev_cpu/recent_used_cpu if they're good wakeup > candidates but not sharing the cluster with the target. > o on scanning failure use the prev_cpu/recent_used_cpu if > they're recorded as idle > > [1] https://lore.kernel.org/all/ZGzDLuVaHR1PAYDt@chenyu5-mobl1/ > > Reported-by: Chen Yu <yu.c.chen@intel.com> > Closes: https://lore.kernel.org/all/ZGsLy83wPIpamy6x@chenyu5-mobl1/ > Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
> --- > kernel/sched/fair.c | 17 ++++++++++++++++- > 1 file changed, 16 insertions(+), 1 deletion(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 02d842df5294..d508d1999ecc 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -7346,7 +7346,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target) > bool has_idle_core = false; > struct sched_domain *sd; > unsigned long task_util, util_min, util_max; > - int i, recent_used_cpu; > + int i, recent_used_cpu, prev_aff = -1; > > /* > * On asymmetric system, update task utilization because we will check > @@ -7379,6 +7379,8 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target) > > if (cpus_share_resources(prev, target)) > return prev; > + > + prev_aff = prev; > } > > /* > @@ -7411,6 +7413,8 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target) > > if (cpus_share_resources(recent_used_cpu, target)) > return recent_used_cpu; > + } else { > + recent_used_cpu = -1; > } > > /* > @@ -7451,6 +7455,17 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target) > if ((unsigned)i < nr_cpumask_bits) > return i; > > + /* > + * For cluster machines which have lower sharing cache like L2 or > + * LLC Tag, we tend to find an idle CPU in the target's cluster > + * first. But prev_cpu or recent_used_cpu may also be a good candidate, > + * use them if possible when no idle CPU found in select_idle_cpu(). > + */ > + if ((unsigned int)prev_aff < nr_cpumask_bits) > + return prev_aff; > + if ((unsigned int)recent_used_cpu < nr_cpumask_bits) > + return recent_used_cpu; > + > return target; > } > > -- > 2.24.0 >
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