Re: [PATCH 2/4] x86: Fix CPUIDLE_FLAG_IRQ_ENABLE leaking timer reprogram

[Frederic Weisbecker]

Le Wed, Nov 15, 2023 at 04:52:32PM +0100, Peter Zijlstra a écrit :
> On Wed, Nov 15, 2023 at 10:13:23AM -0500, Frederic Weisbecker wrote:
> > From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > 
> > intel_idle_irq() re-enables IRQs very early. As a result, an interrupt
> > may fire before mwait() is eventually called. If such an interrupt queues
> > a timer, it may go unnoticed until mwait returns and the idle loop
> > handles the tick re-evaluation. And monitoring TIF_NEED_RESCHED doesn't
> > help because a local timer enqueue doesn't set that flag.
> > 
> > The issue is mitigated by the fact that this idle handler is only invoked
> > for shallow C-states when, presumably, the next tick is supposed to be
> > close enough. There may still be rare cases though when the next tick
> > is far away and the selected C-state is shallow, resulting in a timer
> > getting ignored for a while.
> > 
> > Fix this with using sti_mwait() whose IRQ-reenablement only triggers
> > upon calling mwait(), dealing with the race while keeping the interrupt
> > latency within acceptable bounds.
> > 
> > Fixes: c227233ad64c (intel_idle: enable interrupts before C1 on Xeons)
> > Not-yet-signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> 
> Feel free to change to normal SOB, I'm assuming it actually compiles and
> works by now :-)

Not sure, I might have tested it at some point ;-)

Thanks!