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Subject: Re: perf: add cond_resched() to task_function_call()
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        linux-kernel@vger.kernel.org,
        syzbot+bb4935a5c09b5ff79940@syzkaller.appspotmail.com
References: <20200414190351.16893-1-brho@google.com>
 <20200414204208.GI2483@worktop.programming.kicks-ass.net>
From:   Barret Rhoden <brho@google.com>
Message-ID: <bbec4199-37ef-fda2-b18f-3d485c1d5b70@google.com>
Date:   Tue, 14 Apr 2020 17:16:05 -0400
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On 4/14/20 4:42 PM, Peter Zijlstra wrote:
> On Tue, Apr 14, 2020 at 03:03:51PM -0400, Barret Rhoden wrote:
>> Under rare circumstances, task_function_call() can repeatedly fail and
>> cause a soft lockup.
>>
>> There is a slight race where the process is no longer running on the cpu
>> we targeted by the time remote_function() runs.  The code will simply
>> try again.  If we are very unlucky, this will continue to fail, until a
>> watchdog fires.  This can happen in a heavily loaded, multi-core virtual
>> machine.
> 
> Sigh,.. virt again :/
> 
>> Reported-by: syzbot+bb4935a5c09b5ff79940@syzkaller.appspotmail.com
>> Signed-off-by: Barret Rhoden <brho@google.com>
>> ---
>>   kernel/events/core.c | 12 ++++++++----
>>   1 file changed, 8 insertions(+), 4 deletions(-)
>>
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index 55e44417f66d..65c2c05e24c2 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -99,7 +99,7 @@ static void remote_function(void *data)
>>    *
>>    * returns: @func return value, or
>>    *	    -ESRCH  - when the process isn't running
>> - *	    -EAGAIN - when the process moved away
>> + *	    -ENXIO  - when the cpu the process was on has gone offline
>>    */
> 
> Hurm.. I don't think that was actually intended behaviour.

Yeah, I noticed we never return EAGAIN, so I figured I'd look for any 
other potential return values.

> As long as
> the task lives we ought to retry. Luckily I don't think the current code
> cares much, it'll loop again on the caller side.
> 
> With the exception of perf_cgroup_attach() that is, that might actually
> be broken because of this.
> 
>>   static int
>>   task_function_call(struct task_struct *p, remote_function_f func, void *info)
>> @@ -112,11 +112,15 @@ task_function_call(struct task_struct *p, remote_function_f func, void *info)
>>   	};
>>   	int ret;
>>   
>> -	do {
>> -		ret = smp_call_function_single(task_cpu(p), remote_function, &data, 1);
>> +	while (1) {
>> +		ret = smp_call_function_single(task_cpu(p), remote_function,
>> +					       &data, 1);
>>   		if (!ret)
>>   			ret = data.ret;
>> -	} while (ret == -EAGAIN);
>> +		if (ret != -EAGAIN)
>> +			break;
>> +		cond_resched();
>> +	}
> 
> So how about we make that:
> 
> 	for (;;) {
> 		ret = smp_call_function_single(task_cpu(p), remote_function, &data, 1);
> 		ret = !ret ? data.ret : -EAGAIN;
> 
> 		if (ret != -EAGAIN)
> 			break;
> 
> 		cond_resched();
> 	}
> 
> Or something like that, hmmm?

Can do.  I can put a comment in too.  /* Retry due to any failure in 
smp_call_function */ or something.

Thanks,

Barret