linux-kernel - Re: [PATCH v4 05/16] add_timer_on(): Make sure callers have TIMER

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Date:   Mon, 7 Nov 2022 11:11:15 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Anna-Maria Behnsen <anna-maria@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        John Stultz <jstultz@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Eric Dumazet <edumazet@...gle.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Arjan van de Ven <arjan@...radead.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Rik van Riel <riel@...riel.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Theodore Ts'o <tytso@....edu>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Stephen Boyd <sboyd@...nel.org>, Tejun Heo <tj@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH v4 05/16] add_timer_on(): Make sure callers have
 TIMER_PINNED flag

On Mon, Nov 07, 2022 at 09:11:11AM +0100, Anna-Maria Behnsen wrote:
> On Fri, 4 Nov 2022, Frederic Weisbecker wrote:
> 
> > On Fri, Nov 04, 2022 at 03:57:26PM +0100, Anna-Maria Behnsen wrote:
> > > The implementation of the hierachical timer pull model will change the
> > > timer bases per CPU. Timers, that have to expire on a specific CPU, require
> > > the TIMER_PINNED flag. Otherwise they will be queued on the dedicated CPU
> > > but in global timer base and those timers could also expire on other
> > > CPUs. Timers with TIMER_DEFERRABLE flag end up in a separate base anyway
> > > and are executed on the local CPU only.
> > > 
> > > Therefore add the missing TIMER_PINNED flag for those callers who use
> > > add_timer_on() without the flag. No functional change.
> > 
> > You're fixing the current callers but what about the future ones?
> > 
> > add_timer_on() should always guarantee that a timer runs on the
> > right destination, which is not the case after your patchset if the
> > timer hasn't been set to TIMER_PINNED.
> > 
> > Therefore I think we should either have:
> > 
> > * add_timer_on() enforce TIMER_PINNED (doesn't work because if the timer is
> >   later called with mod_timer(), we should expect it to run anywhere)
> > 
> > or
> > 
> > * add_timer_on() warns if !TIMER_PINNED
> 
> This is already part of the last patch of the queue where also the
> crystalball logic is removed. But the patch where I added the WARN_ONCE()
> might be the wrong patch, it should be better part of the next patch where
> the new timer bases are introduced.

Ok.

> 
> > or
> > 
> > * have an internal flag TIMER_LOCAL, that is turned on when
> >   add_timer_on() is called or add_timer()/mod_timer() is called
> >   on a TIMER_PINNED. Otherwise it is turned off.
> > 
> > The last solution should work with existing API and you don't need to
> > chase the current and future users of add_timer_on().
> 
> With the last approach it doesn't matter how the timer is setup. Everything
> is done by timer code implicitly. When a future caller uses add_timer_on()
> and wants to modfiy this "implicitly pinned timer", he will call
> mod_timer() and the timer is no longer pinned (if it do not end up in the
> same bucket it was before). For a user this does not seems to be very
> obvious, or am I wrong?

That's right indeed.

> 
> But if the caller sets up the timer correctly we do not need this extra
> timer flag. With the WARN_ONCE() in place, callers need to do the timer
> setup properly and it is more clear to the caller what should be done.

Yeah that sounds better.

> BTW, the hunk in this patch for the workqueue is also not a final fix in my
> opinion. I'm preparing a cleanup queue (it's part of the deferrable cleanup
> queue), where I want to set the timer flags properly when
> initializing/defining the workers. I should have added a comment here...

Ok, if we have some pinned initializers such as DECLARE_DELAYED_WORK_PINNED()
and DECLARE_DEFERRABKE_WORK_PINNED() then I think that cleans the situation.

Sounds good, thanks!

> 
> Thanks,
> 
> 	Anna-Maria
>