Re: [PATCH 0/2] net: Use synchronous wakeups selectively

[Srikar Dronamraju]

* Jakub Kicinski <kuba@xxxxxxxxxx> [2026-07-22 10:08:42]:

Hi Jakub, Thanks for taking a look.

> On Tue, 14 Jul 2026 07:09:41 +0530 Srikar Dronamraju wrote:
> > The scheduler assumes in several wakeup paths that a task using WF_SYNC
> > is likely to yield the CPU shortly. However several networking wakeup
> > paths unconditionally use synchronous wakeups even when the waking task
> > continues execution.
> > 
> > During wakeup, with WF_SYNC flag set, because of the assumption that
> > current thread is ready to give up, the wakee thread will be migrated
> > from any core within the chip to the current LLC. If these operations
> > are frequent, and wakers are actually not going away, then it will lead
> > to load imbalance and hurt performance. This is especially true in
> > architectures where LLCs are small and number of LLCs per chip are more.
> > 
> > Running vllm workload was run on Power10 system
> >                                   No patch      with patch %diff
> > Inference Time (sec)              17.84         16.35     -8.35%
> > llm query bandwidth (tokens/sec)  14.78         16.21     +9.68%
> > 
> > Lower inference time and higher tokens/sec is better.
> 
> Somewhat related recent patch:
> 
> https://lore.kernel.org/all/20260708133815.3419465-1-usama.arif@xxxxxxxxx/
> 
> Maybe other maintainers will chime in, but I'm not convinced 
> by the heuristics you're adding. The behavior will be workload
> specific.

Currently scheduler provides 2 APIs because it supports 2 subtle behaviours.
1st API being wake_up_interruptible_poll and 2nd API being
wake_up_interruptible_sync_poll. The reason scheduler provides 2 APIs is
because the scheduler cant decide which behaviour the callee wants.

If the scheduler could have figured out which behaviour then there would
have been only 1 API. So the onus of using the right API is on the
callee.

Now the behaviour between the 2 APIs depends on fact that the callee is
going to give up the CPU now or if the callee will continue to run. If the
callee is going to continue to run, its better to use the more generic API
of the two which is wake_up_interruptible_poll. However if the callee is
giving up the CPU, its better to use wake_up_interruptible_sync_poll.

@peterz @ingom can you please confirm this behaviour is inline with
scheduler maintainers view.

Currently the networking code as I see is unconditionally (or blindly)
asking for sync behaviour i.e the socket API is saying the callees are going
to give up CPU soon. However the problem I am seeing is the callees are
actually not giving up CPU.

> The extent of networking involvement should be letting
> the scheduler know that we're waking from IO, not leaking scheduler
> heuristics into networking.

In this change, what we are depending on is file's flag that the socket
request is blocking or non-blocking. So why do we say we are leaking
scheduler hueristics? Would using an API or another API mean leaking
scheduler hueristics?  If we are blocking, this change will use
wake_up_interruptible_sync_poll and if non-blocking, it will use
wake_up_interruptible_poll.

-- 
Thanks and Regards
Srikar Dronamraju