From: Michal Hocko mhocko@suse.com

[ Upstream commit f01f17d3705bb6081c9e5728078f64067982be36 ]

Mike has reported a considerable overhead of refresh_cpu_vm_stats from the idle entry during pipe test:

12.89% [kernel] [k] refresh_cpu_vm_stats.isra.12 4.75% [kernel] [k] __schedule 4.70% [kernel] [k] mutex_unlock 3.14% [kernel] [k] __switch_to

This is caused by commit 0eb77e988032 ("vmstat: make vmstat_updater deferrable again and shut down on idle") which has placed quiet_vmstat into cpu_idle_loop. The main reason here seems to be that the idle entry has to get over all zones and perform atomic operations for each vmstat entry even though there might be no per cpu diffs. This is a pointless overhead for _each_ idle entry.

Make sure that quiet_vmstat is as light as possible.

First of all it doesn't make any sense to do any local sync if the current cpu is already set in oncpu_stat_off because vmstat_update puts itself there only if there is nothing to do.

Then we can check need_update which should be a cheap way to check for potential per-cpu diffs and only then do refresh_cpu_vm_stats.

The original patch also did cancel_delayed_work which we are not doing here. There are two reasons for that. Firstly cancel_delayed_work from idle context will blow up on RT kernels (reported by Mike):

CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.5.0-rt3 #7 Hardware name: MEDION MS-7848/MS-7848, BIOS M7848W08.20C 09/23/2013 Call Trace: dump_stack+0x49/0x67 ___might_sleep+0xf5/0x180 rt_spin_lock+0x20/0x50 try_to_grab_pending+0x69/0x240 cancel_delayed_work+0x26/0xe0 quiet_vmstat+0x75/0xa0 cpu_idle_loop+0x38/0x3e0 cpu_startup_entry+0x13/0x20 start_secondary+0x114/0x140

And secondly, even on !RT kernels it might add some non trivial overhead which is not necessary. Even if the vmstat worker wakes up and preempts idle then it will be most likely a single shot noop because the stats were already synced and so it would end up on the oncpu_stat_off anyway. We just need to teach both vmstat_shepherd and vmstat_update to stop scheduling the worker if there is nothing to do.

[mgalbraith@suse.de: cancel pending work of the cpu_stat_off CPU] Signed-off-by: Michal Hocko mhocko@suse.com Reported-by: Mike Galbraith umgwanakikbuti@gmail.com Acked-by: Christoph Lameter cl@linux.com Signed-off-by: Mike Galbraith mgalbraith@suse.de Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Daniel Wagner wagi@monom.org --- Hi Greg,

Upstream commmit 0eb77e988032 ("vmstat: make vmstat_updater deferrable again and shut down on idle") was back ported in v4.4.178 (bdf3c006b9a2). For -rt we definitely need the bugfix f01f17d3705b ("mm, vmstat: make quiet_vmstat lighter") as well.

Since the offending patch was back ported to v4.4 stable only, the other stable branches don't need an update (offending patch and bug fix are already in).

Could you please queue the above patch for v4.4.y?

Thanks, Daniel

mm/vmstat.c | 68 ++++++++++++++++++++++++++++++++++++----------------- 1 file changed, 46 insertions(+), 22 deletions(-)

diff --git a/mm/vmstat.c b/mm/vmstat.c index d0ff5de06bfa..929f1fc3a4b1 100644 --- a/mm/vmstat.c +++ b/mm/vmstat.c @@ -1406,10 +1406,15 @@ static void vmstat_update(struct work_struct *w) * Counters were updated so we expect more updates * to occur in the future. Keep on running the * update worker thread. + * If we were marked on cpu_stat_off clear the flag + * so that vmstat_shepherd doesn't schedule us again. */ - queue_delayed_work_on(smp_processor_id(), vmstat_wq, - this_cpu_ptr(&vmstat_work), - round_jiffies_relative(sysctl_stat_interval)); + if (!cpumask_test_and_clear_cpu(smp_processor_id(), + cpu_stat_off)) { + queue_delayed_work_on(smp_processor_id(), vmstat_wq, + this_cpu_ptr(&vmstat_work), + round_jiffies_relative(sysctl_stat_interval)); + } } else { /* * We did not update any counters so the app may be in @@ -1437,18 +1442,6 @@ static void vmstat_update(struct work_struct *w) * until the diffs stay at zero. The function is used by NOHZ and can only be * invoked when tick processing is not active. */ -void quiet_vmstat(void) -{ - if (system_state != SYSTEM_RUNNING) - return; - - do { - if (!cpumask_test_and_set_cpu(smp_processor_id(), cpu_stat_off)) - cancel_delayed_work(this_cpu_ptr(&vmstat_work)); - - } while (refresh_cpu_vm_stats(false)); -} - /* * Check if the diffs for a certain cpu indicate that * an update is needed. @@ -1472,6 +1465,30 @@ static bool need_update(int cpu) return false; }

+void quiet_vmstat(void) +{ + if (system_state != SYSTEM_RUNNING) + return; + + /* + * If we are already in hands of the shepherd then there + * is nothing for us to do here. + */ + if (cpumask_test_and_set_cpu(smp_processor_id(), cpu_stat_off)) + return; + + if (!need_update(smp_processor_id())) + return; + + /* + * Just refresh counters and do not care about the pending delayed + * vmstat_update. It doesn't fire that often to matter and canceling + * it would be too expensive from this path. + * vmstat_shepherd will take care about that for us. + */ + refresh_cpu_vm_stats(false); +} +

/* * Shepherd worker thread that checks the @@ -1489,18 +1506,25 @@ static void vmstat_shepherd(struct work_struct *w)

get_online_cpus(); /* Check processors whose vmstat worker threads have been disabled */ - for_each_cpu(cpu, cpu_stat_off) - if (need_update(cpu) && - cpumask_test_and_clear_cpu(cpu, cpu_stat_off)) - - queue_delayed_work_on(cpu, vmstat_wq, - &per_cpu(vmstat_work, cpu), 0); + for_each_cpu(cpu, cpu_stat_off) { + struct delayed_work *dw = &per_cpu(vmstat_work, cpu);

+ if (need_update(cpu)) { + if (cpumask_test_and_clear_cpu(cpu, cpu_stat_off)) + queue_delayed_work_on(cpu, vmstat_wq, dw, 0); + } else { + /* + * Cancel the work if quiet_vmstat has put this + * cpu on cpu_stat_off because the work item might + * be still scheduled + */ + cancel_delayed_work(dw); + } + } put_online_cpus();

schedule_delayed_work(&shepherd, round_jiffies_relative(sysctl_stat_interval)); - }

static void __init start_shepherd_timer(void)