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

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Message-ID: <878qu1qv7i.ffs@tglx>
Date: Sat, 02 Nov 2024 20:41:53 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Frederic Weisbecker <frederic@...nel.org>
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

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.

Thanks,

        tglx