Re: [PATCH v2 2/3] sched/fair: Move hot load_avg into its own cacheline

Waiman Long
2015-12-03T19:56:42Z

On 12/03/2015 06:12 AM, Peter Zijlstra wrote:
>
> I made this:
>
> ---
> Subject: sched/fair: Move hot load_avg into its own cacheline
> From: Waiman Long<Waiman.Long@hpe.com>
> Date: Wed, 2 Dec 2015 13:41:49 -0500
>
> If a system with large number of sockets was driven to full
> utilization, it was found that the clock tick handling occupied a
> rather significant proportion of CPU time when fair group scheduling
> and autogroup were enabled.
>
> Running a java benchmark on a 16-socket IvyBridge-EX system, the perf
> profile looked like:
>
> 10.52% 0.00% java [kernel.vmlinux] [k] smp_apic_timer_interrupt
> 9.66% 0.05% java [kernel.vmlinux] [k] hrtimer_interrupt
> 8.65% 0.03% java [kernel.vmlinux] [k] tick_sched_timer
> 8.56% 0.00% java [kernel.vmlinux] [k] update_process_times
> 8.07% 0.03% java [kernel.vmlinux] [k] scheduler_tick
> 6.91% 1.78% java [kernel.vmlinux] [k] task_tick_fair
> 5.24% 5.04% java [kernel.vmlinux] [k] update_cfs_shares
>
> In particular, the high CPU time consumed by update_cfs_shares()
> was mostly due to contention on the cacheline that contained the
> task_group's load_avg statistical counter. This cacheline may also
> contains variables like shares, cfs_rq& se which are accessed rather
> frequently during clock tick processing.
>
> This patch moves the load_avg variable into another cacheline
> separated from the other frequently accessed variables. It also
> creates a cacheline aligned kmemcache for task_group to make sure
> that all the allocated task_group's are cacheline aligned.
>
> By doing so, the perf profile became:
>
> 9.44% 0.00% java [kernel.vmlinux] [k] smp_apic_timer_interrupt
> 8.74% 0.01% java [kernel.vmlinux] [k] hrtimer_interrupt
> 7.83% 0.03% java [kernel.vmlinux] [k] tick_sched_timer
> 7.74% 0.00% java [kernel.vmlinux] [k] update_process_times
> 7.27% 0.03% java [kernel.vmlinux] [k] scheduler_tick
> 5.94% 1.74% java [kernel.vmlinux] [k] task_tick_fair
> 4.15% 3.92% java [kernel.vmlinux] [k] update_cfs_shares
>
> The %cpu time is still pretty high, but it is better than before. The
> benchmark results before and after the patch was as follows:
>
> Before patch - Max-jOPs: 907533 Critical-jOps: 134877
> After patch - Max-jOPs: 916011 Critical-jOps: 142366
>
> Cc: Scott J Norton<scott.norton@hpe.com>
> Cc: Douglas Hatch<doug.hatch@hpe.com>
> Cc: Ingo Molnar<mingo@redhat.com>
> Cc: Yuyang Du<yuyang.du@intel.com>
> Cc: Paul Turner<pjt@google.com>
> Cc: Ben Segall<bsegall@google.com>
> Cc: Morten Rasmussen<morten.rasmussen@arm.com>
> Signed-off-by: Waiman Long<Waiman.Long@hpe.com>
> Signed-off-by: Peter Zijlstra (Intel)<peterz@infradead.org>
> Link: http://lkml.kernel.org/r/1449081710-20185-3-git-send-email-Waiman.Long@hpe.com
> ---
> kernel/sched/core.c | 10 +++++++---
> kernel/sched/sched.h | 7 ++++++-
> 2 files changed, 13 insertions(+), 4 deletions(-)
>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7345,6 +7345,9 @@ int in_sched_functions(unsigned long add
> */
> struct task_group root_task_group;
> LIST_HEAD(task_groups);
> +
> +/* Cacheline aligned slab cache for task_group */
> +static struct kmem_cache *task_group_cache __read_mostly;
> #endif
>
> DECLARE_PER_CPU(cpumask_var_t, load_balance_mask);
> @@ -7402,11 +7405,12 @@ void __init sched_init(void)
> #endif /* CONFIG_RT_GROUP_SCHED */
>
> #ifdef CONFIG_CGROUP_SCHED
> + task_group_cache = KMEM_CACHE(task_group, 0);
> +
Thanks for making that change.

Do we need to add the flag SLAB_HWCACHE_ALIGN? Or we could make a helper
flag that define SLAB_HWCACHE_ALIGN if CONFIG_FAIR_GROUP_SCHED is
defined. Other than that, I am fine with the change.

Cheers,
Longman