Messages in this thread | | | Date | Sun, 8 Oct 2023 17:35:35 +0000 | From | Joel Fernandes <> | Subject | Re: [PATCH RFC] sched/fair: Avoid unnecessary IPIs for ILB |
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On Fri, Oct 06, 2023 at 12:51:41PM +0200, Ingo Molnar wrote: > > * Joel Fernandes (Google) <joel@joelfernandes.org> 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. ] > > > > 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))) > > Ok, judging by your IPI reduction numbers this is definitely an > optimization we want to do. > > The patch does make _nohz_idle_balance() run more parallel, as previously > it would be generally run by the first-idle CPU in nohz.idle_cpus_mask (at > least for next_balance updates), but I think it's still SMP-safe, as all > key data structure updates are already rq-locked AFAICS.
One thing I am confused about in the original code is:
tick_nohz_idle_stop_tick() is what sets the nohz.idle_cpus_mask. However, nohz_run_idle_balance() is called before that can happen, in do_idle(). So it is possible that NOHZ_NEWILB_KICK is set for a CPU but it is not yet in the mask.
So will this code in _nohz_idle_balance() really run in such a scenario?
if (flags & NOHZ_STATS_KICK) has_blocked_load |= update_nohz_stats(rq);
AFAICS, this loop may not select the CPU due to its absence from the mask: for_each_cpu_wrap(balance_cpu, nohz.idle_cpus_mask, this_cpu+1)
I must be missing something. I'll go trace this path later as well.
thanks,
- Joel
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