Re: [PATCH net] net: fix race between napi kthread mode and busy poll

[Wei Wang <weiwan@google.com>]

On Wed, Feb 24, 2021 at 4:33 PM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Wed, 24 Feb 2021 16:16:58 -0800 Wei Wang wrote:
> > On Wed, Feb 24, 2021 at 4:11 PM Alexander Duyck <alexanderduyck@fb.com> wrote:
> > >
> > > The problem with adding a bit for SCHED_THREADED is that you would
> > > have to heavily modify napi_schedule_prep so that it would add the
> > > bit. That is the reason for going with adding the bit to the busy
> > > poll logic because it added no additional overhead. Adding another
> > > atomic bit setting operation or heavily modifying the existing one
> > > would add considerable overhead as it is either adding a
> > > complicated conditional check to all NAPI calls, or adding an
> > > atomic operation to the path for the threaded NAPI.
> >
> > Please help hold on to the patch for now. I think Martin is still
> > seeing issues on his setup even with this patch applied. I have not
> > yet figured out why. But I think we should not merge this patch until
> > the issue is cleared. Will update this thread with progress.
>
> If I'm looking right __busy_poll_stop() is only called if the last
> napi poll used to re-enable IRQs consumed full budget. You need to
> clear your new bit in busy_poll_stop(), not in __busy_poll_stop().
> That will fix the case when hand off back to the normal poller (sirq,
> or thread) happens without going thru __napi_schedule().

If the budget is not fully consumed, napi_complete_done() should have
been called by the driver which will clear SCHED_BUSY_POLL bit.