linux-kernel - Re: [patch v6 02/20] posix-timers: Make signal overrun accounting sensible

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Message-ID: <Zyauaj2icYHqc33N@pavilion.home>
Date: Sat, 2 Nov 2024 23:57:46 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	John Stultz <jstultz@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, Stephen Boyd <sboyd@...nel.org>,
	Eric Biederman <ebiederm@...ssion.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [patch v6 02/20] posix-timers: Make signal overrun accounting
 sensible

Le Sat, Nov 02, 2024 at 08:41:53PM +0100, Thomas Gleixner a écrit :
> On Fri, Nov 01 2024 at 21:36, Thomas Gleixner wrote:
> > On Fri, Nov 01 2024 at 13:51, Frederic Weisbecker wrote:
> >> Le Thu, Oct 31, 2024 at 04:46:25PM +0100, Thomas Gleixner a écrit :
> >>> @@ -1968,15 +1968,9 @@ int send_sigqueue(struct sigqueue *q, st
> >>>  
> >>>  	ret = 0;
> >>>  	if (unlikely(!list_empty(&q->list))) {
> >>> -		/*
> >>> -		 * If an SI_TIMER entry is already queue just increment
> >>> -		 * the overrun count.
> >>> -		 */
> >>> -		q->info.si_overrun++;
> >>>  		result = TRACE_SIGNAL_ALREADY_PENDING;
> >>>  		goto out;
> >>>  	}
> >>> -	q->info.si_overrun = 0;
> >>
> >> So it's not cleared anymore on signal queue?
> >>
> >> Not sure if it's a big problem but if an interval timer gets a signal with
> >> overruns and then the timer is reset later as non interval, the resulting
> >> upcoming signals will still carry the previous non-zero overruns?
> >
> > Duh. Yes.
> >
> >> However it's better to keep the overrun update on a single place so
> >> perhaps this?
> >>
> >> diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
> >> index 66ed49efc02f..f06c52731d65 100644
> >> --- a/kernel/time/posix-timers.c
> >> +++ b/kernel/time/posix-timers.c
> >> @@ -282,6 +282,8 @@ bool posixtimer_deliver_signal(struct kernel_siginfo *info)
> >>  		++timr->it_signal_seq;
> >>  
> >>  		info->si_overrun = timer_overrun_to_int(timr);
> >> +	} else {
> >> +		info->si_overrun = 0;
> >>  	}
> >>  	ret = true;
> >>  
> >> Other than that:
> >
> > Let me fold that.
> 
> Actually no. info is the siginfo which was allocated by the signal
> delivery code on stack.
> 
> collect_signal() copies timer->sigqueue.info into that siginfo
> struct. As timer->sigqueue.info.si_overrun is zero and never written to,
> this else path is pointless.

Good point, thanks for pointing out!