linux-kernel - Re: [RFC PATCH v2 10/12] rv: Retry when da monitor detects race conditions

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Message-ID: <20250521065800.jgtPyWd7@linutronix.de>
Date: Wed, 21 May 2025 08:58:00 +0200
From: Nam Cao <namcao@...utronix.de>
To: Gabriele Monaco <gmonaco@...hat.com>
Cc: linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	linux-trace-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Tomas Glozar <tglozar@...hat.com>, Juri Lelli <jlelli@...hat.com>
Subject: Re: [RFC PATCH v2 10/12] rv: Retry when da monitor detects race
 conditions

On Mon, May 19, 2025 at 01:13:01PM +0200, Gabriele Monaco wrote:
> 
> 
> On Mon, 2025-05-19 at 12:38 +0200, Nam Cao wrote:
> > On Mon, May 19, 2025 at 12:28:12PM +0200, Gabriele Monaco wrote:
> > > Mmh, although definitely unlikely, I'm thinking of a case in which
> > > the
> > > event starts on one CPU and at the same time we see events in IRQ
> > > and 
> > > on another CPU, let's say continuously. Nothing forbids that
> > > between
> > > any two consecutive try_cmpxchg another CPU/context changes the
> > > next
> > > state (making the local try_cmpxchg fail).
> > > In practice I've never seen it going on the second iteration, as
> > > the
> > > critical section is really tiny, but I'm not sure we can guarantee
> > > this
> > > never happens.
> > > Or am I missing something?
> > 
> > I have a feeling that you missed my point. I agree that the retrying
> > is
> > needed, because we may race with another.
> > 
> > What I am proposing is that we drop the MAX_DA_RETRY_RACING_EVENTS,
> > and
> > just keep retrying until we succeed.
> > 
> > And that's safe to do, because the maximum number of retries is the
> > number
> > of tasks contending with us to set the monitor's state. So we know we
> > won't
> > be retrying for long.
> 
> I get this point, what I mean is: can we really guarantee the number of
> contending tasks (or contexts) is finite?
> In other words, the try_cmpxchg guarantees 1 and only 1 actor wins
> every time, but cannot guarantee all actors will eventually win, an
> actor /could/ be hanging there forever.
> 
> This handler is running for each event in the monitor and tracepoint
> handlers can be interrupted as well as run in interrupt context (where
> of course they cannot be interrupted). I don't think the number of
> actors is bounded by the number of CPUs.
> I see this situation is extremely unlikely, but in an exotic scenario
> where a CPU is sufficiently slower than others (e.g. in a VM) I believe
> we can see this critical section large enough for this to potentially
> happen.
> 
> I'm not quite afraid of infinite loops, but rather RV introducing
> unbounded latency very hard to track and without any reporting.
> Chances are, since tracepoints and actual traced events are not atomic,
> that by the time this delayed context /wins/ the RV event is no longer
> current, so we may see an error already.
> 
> Does it make sense to you or am I making it more complex than it should
> be?

Right, I can see that being a problem. But I don't know enough about it to
comment further, so do as you think best, maybe someone else can help.

Best regards,
Nam