Messages in this thread | | | Date | Sun, 8 Oct 2023 16:50:20 +0000 | From | Joel Fernandes <> | Subject | Re: [PATCH RFC] sched/fair: Avoid unnecessary IPIs for ILB |
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On Sat, Oct 07, 2023 at 01:40:53AM +0530, Shrikanth Hegde wrote: > > > On 10/7/23 12:48 AM, Shrikanth Hegde wrote: > > > > > > On 10/5/23 9:47 PM, Joel Fernandes (Google) wrote: > >> From: Vineeth Pillai <vineethrp@google.com> > >> > >> Whenever a CPU stops its tick, it now requires another idle CPU to handle the > >> balancing for it because it can't perform its own periodic load balancing. > >> This means it might need to update 'nohz.next_balance' to 'rq->next_balance' if > >> the upcoming nohz-idle load balancing is too distant in the future. This update > >> process is done by triggering an ILB, as the general ILB handler > >> (_nohz_idle_balance) that manages regular nohz balancing also refreshes > >> 'nohz.next_balance' by looking at the 'rq->next_balance' of all other idle CPUs > >> and selecting the smallest value. > >> > >> Triggering this ILB can be achieved by setting the NOHZ_NEXT_KICK flag. This > >> primarily results in the ILB handler updating 'nohz.next_balance' while > >> possibly not doing any load balancing at all. However, sending an IPI merely to > >> refresh 'nohz.next_balance' seems excessive, and there ought to be a more > >> efficient method to update 'nohz.next_balance' from the local CPU. > >> > >> Fortunately, there already exists a mechanism to directly invoke the ILB > >> handler (_nohz_idle_balance) without initiating an IPI. It's accomplished by > >> setting the NOHZ_NEWILB_KICK flag. This flag is set during regular "newly idle" > >> balancing and solely exists to update a CPU's blocked load if it couldn't pull > >> more tasks during regular "newly idle balancing" - and it does so without > >> having to send any IPIs. Once the flag is set, the ILB handler is called > >> directly from do_idle()-> nohz_run_idle_balance(). While its goal is to update > >> the blocked load without an IPI, in our situation, we aim to refresh > >> 'nohz.next_balance' without an IPI but we can piggy back on this. > >> > >> So in this patch, we reuse this mechanism by also setting the NOHZ_NEXT_KICK to > >> indicate nohz.next_balance needs an update via this direct call shortcut. Note > >> that we set this flag without knowledge that the tick is about to be stopped, > >> because at the point we do it, we have no way of knowing that. However we do > >> know that the CPU is about to enter idle. In our testing, the reduction in IPIs > >> is well worth updating nohz.next_balance a few more times. > >> > >> Also just to note, without this patch we observe the following pattern: > >> > >> 1. A CPU is about to stop its tick. > >> 2. It sets nohz.needs_update to 1. > >> 3. It then stops its tick and goes idle. > >> 4. The scheduler tick on another CPU checks this flag and decides an ILB kick is needed. > >> 5. The ILB CPU ends up being the one that just stopped its tick! > >> 6. This results in an IPI to the tick-stopped CPU which ends up waking it up > >> and disturbing it! > >> > >> Testing shows a considerable reduction in IPIs when doing this: > >> > >> Running "cyclictest -i 100 -d 100 --latency=1000 -t -m" on a 4vcpu VM > >> the IPI call count profiled over 10s period is as follows: > >> without fix: ~10500 > >> with fix: ~1000 > >> > >> Fixes: 7fd7a9e0caba ("sched/fair: Trigger nohz.next_balance updates when a CPU goes NOHZ-idle") > >> > >> [ Joel: wrote commit messages, collaborated on fix, helped reproduce issue etc. ] > > > > Hi Joel/Vineeth. > > > > Its an interesting patch. > > > > Gave this patch a try on powerpc system with 96 CPU. (12 cores SMT8) > > Was debugging an issue where ILB count goes up significantly at a specific > > busy CPU count. Haven't yet found out why. Its WIP. Was curious if this patch > > would address that issue. > > > > cloned rt-test repo and ran same cyclictest command and collected > > softirq's count using bcc tool. That count remains same more or less with patch. > > Is what I am checking incorrect? Any other way to check IPI count? > > > > base 6.6_rc4 +patch > > > > block 31.00 48.86 > > net_rx 475.90 348.90 > > timer 2213.20 2405.00 > > rcu 33057.30 34738.10 > > sched 175904.70 169695.60 > > > > Ah, there is hardirq which shows IPI count. Didnt think of it. > This is average of 10 runs where hardirq was collected at 10s while running cyclictest. > This shows nice improvement. in base6.6 there were few instance where > number of IPI was much high. > > base 6.6_rc4 +patch > IPI-1 2741.1 1382.3 >
Very cool! So I'll go ahead and add this data as well for the next revision.
(I hope to post a new version in a few days after addressing all the review comments, I am unfortunately a bit slow this week due to travel and other things).
Thanks,
- Joel
> > > > >> > >> Cc: Suleiman Souhlal <suleiman@google.com> > >> Cc: Steven Rostedt <rostedt@goodmis.org> > >> Cc: Hsin Yi <hsinyi@google.com> > >> Cc: Frederic Weisbecker <frederic@kernel.org> > >> Cc: Paul E. McKenney <paulmck@kernel.org> > >> Signed-off-by: Vineeth Pillai <vineethrp@google.com> > >> Co-developed-by: Joel Fernandes (Google) <joel@joelfernandes.org> > >> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> > >> --- > >> kernel/sched/fair.c | 21 ++++++++++++++------- > >> 1 file changed, 14 insertions(+), 7 deletions(-) > >> > >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > >> index cb225921bbca..2ece55f32782 100644 > >> --- a/kernel/sched/fair.c > >> +++ b/kernel/sched/fair.c > >> @@ -11786,13 +11786,12 @@ void nohz_balance_enter_idle(int cpu) > >> /* > >> * Ensures that if nohz_idle_balance() fails to observe our > >> * @idle_cpus_mask store, it must observe the @has_blocked > >> - * and @needs_update stores. > >> + * stores. > >> */ > >> smp_mb__after_atomic(); > >> > >> set_cpu_sd_state_idle(cpu); > >> > >> - WRITE_ONCE(nohz.needs_update, 1); > >> out: > >> /* > >> * Each time a cpu enter idle, we assume that it has blocked load and > >> @@ -11945,21 +11944,25 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) > >> } > >> > >> /* > >> - * Check if we need to run the ILB for updating blocked load before entering > >> - * idle state. > >> + * Check if we need to run the ILB for updating blocked load and/or updating > >> + * nohz.next_balance before entering idle state. > >> */ > >> void nohz_run_idle_balance(int cpu) > >> { > >> unsigned int flags; > >> > >> - flags = atomic_fetch_andnot(NOHZ_NEWILB_KICK, nohz_flags(cpu)); > >> + flags = atomic_fetch_andnot(NOHZ_NEWILB_KICK | NOHZ_NEXT_KICK, nohz_flags(cpu)); > >> + > >> + if (!flags) > >> + return; > >> > >> /* > >> * Update the blocked load only if no SCHED_SOFTIRQ is about to happen > >> * (ie NOHZ_STATS_KICK set) and will do the same. > >> */ > >> - if ((flags == NOHZ_NEWILB_KICK) && !need_resched()) > >> - _nohz_idle_balance(cpu_rq(cpu), NOHZ_STATS_KICK); > >> + if ((flags == (flags & (NOHZ_NEXT_KICK | NOHZ_NEWILB_KICK))) && > >> + !need_resched()) > >> + _nohz_idle_balance(cpu_rq(cpu), flags); > >> } > >> > >> static void nohz_newidle_balance(struct rq *this_rq) > >> @@ -11977,6 +11980,10 @@ static void nohz_newidle_balance(struct rq *this_rq) > >> if (this_rq->avg_idle < sysctl_sched_migration_cost) > >> return; > >> > >> + /* If rq->next_balance before nohz.next_balance, trigger ILB */ > >> + if (time_before(this_rq->next_balance, READ_ONCE(nohz.next_balance))) > >> + atomic_or(NOHZ_NEXT_KICK, nohz_flags(this_cpu)); > >> + > >> /* Don't need to update blocked load of idle CPUs*/ > >> if (!READ_ONCE(nohz.has_blocked) || > >> time_before(jiffies, READ_ONCE(nohz.next_blocked)))
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