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

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Date:   Fri, 4 Nov 2022 17:43:42 +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 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

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().

Thanks.