Re: [PATCH RFC] sched/fair: Avoid unnecessary IPIs for ILB

[Peter Zijlstra]

On Thu, Oct 05, 2023 at 04:17:26PM +0000, Joel Fernandes (Google) wrote:
> From: Vineeth Pillai <vineethrp@xxxxxxxxxx>
> 
> 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")

Hurm.. does this really warrant a Fixes tag? Afaict nothing is currently
broken -- this is a pure optimization question, no?