Re: [PATCH v4 05/16] add_timer_on(): Make sure callers have TIMER_PINNED flag

From: Frederic Weisbecker <frederic@kernel.org>
To: Anna-Maria Behnsen <anna-maria@linutronix.de>
Cc: linux-kernel@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	John Stultz <jstultz@google.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Eric Dumazet <edumazet@google.com>,
	"Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
	Arjan van de Ven <arjan@infradead.org>,
	"Paul E . McKenney" <paulmck@kernel.org>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Rik van Riel <riel@surriel.com>, Ingo Molnar <mingo@redhat.com>,
	Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Theodore Ts'o <tytso@mit.edu>,
	"Jason A. Donenfeld" <Jason@zx2c4.com>,
	Stephen Boyd <sboyd@kernel.org>, Tejun Heo <tj@kernel.org>,
	Lai Jiangshan <jiangshanlai@gmail.com>
Subject: Re: [PATCH v4 05/16] add_timer_on(): Make sure callers have TIMER_PINNED flag
Date: Fri, 4 Nov 2022 17:43:42 +0100	[thread overview]
Message-ID: <20221104164342.GA1440400@lothringen> (raw)
In-Reply-To: <20221104145737.71236-6-anna-maria@linutronix.de>

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.