Re: SMTC lookup in smtc_distribute_timer

["Kevin D. Kissell" <kevink@paralogos.com>]

Rather than just assume all is well, I really would appreciate it
of you could send a positive acknowledgement that it solves
the problem without causing the universe to implode, so that
Ralf can queue up the patch for the repository.

       Regards,

       Kevin K.

Mikael Starvik wrote:
> Yes, I thought of that variant after I sent the email yesterday.
> I'll change our local implementation. If you don't hear anything
> it works as expected in our case (it was pretty easy for us to
> repeat).
>
> /Mikael 
>
> -----Original Message-----
> From: Kevin D. Kissell [mailto:kevink@paralogos.com] 
> Sent: den 10 november 2009 20:46
> To: Mikael Starvik
> Cc: linux-mips@linux-mips.org; Jesper Nilsson
> Subject: Re: SMTC lookup in smtc_distribute_timer
>
> Your failure scenario looks plausible. Mea culpa.  However, I think that
> a more elegant and slightly smaller (depending on just how good
> the optimizer is) fix would be:
>
> diff --git a/arch/mips/kernel/cevt-smtc.c b/arch/mips/kernel/cevt-smtc.c
> index 98bd7de..b102e4f 100644
> --- a/arch/mips/kernel/cevt-smtc.c
> +++ b/arch/mips/kernel/cevt-smtc.c
> @@ -173,11 +173,12 @@ void smtc_distribute_timer(int vpe)
>         unsigned int mtflags;
>         int cpu;
>         struct clock_event_device *cd;
> -       unsigned long nextstamp = 0L;
> +       unsigned long nextstamp;
>         unsigned long reference;
>  
>  
>  repeat:
> +       nextstamp = 0L;
>         for_each_online_cpu(cpu) {
>             /*
>              * Find virtual CPUs within the current VPE who have
>
>
>
> I don't have access to SMTC-capable hardware just now, but
> I guess the way to test this would be to have a test program
> or kernel test stub program two events separated by the smallest
> possible increment, so that the second will have passed by the
> time interrupt services for the first.
>
>           Regards,
>
>           Kevin K.
>
> Mikael Starvik wrote:
>   
>> Ok, my guess is something like this:
>>
>> 1. At the end of smtc_distribute_timer, nextstamp is valid and has already 
>> passed so we goto repeat. 
>> 2. Nothing updates nextstamp (only updated if the timeout is in the future 
>> And we just decided it is in the past)
>> 3. At the end nextstamp still has the same value so it is still valid and
>> in the past.
>> 4. This repeats until read_c0_count has a value which causes nextstamp to
>> be in the future.
>>
>> One possible patch that seams to solve it for me below. This is probably 
>> not the correct solution so I'll need help from the SMTC experts to review
>> it and come up with the correct solution.
>>
>> Best Regards
>> /Mikael
>>
>> Index: cevt-smtc.c
>> ===================================================================
>> RCS file: /usr/local/cvs/linux/os/linux-2.6/arch/mips/kernel/cevt-smtc.c,v
>> retrieving revision 1.2
>> diff -u -r1.2 cevt-smtc.c
>> --- cevt-smtc.c	2 Sep 2009 10:07:51 -0000	1.2
>> +++ cevt-smtc.c	10 Nov 2009 11:40:31 -0000
>> @@ -223,8 +223,10 @@
>>  		write_c0_compare(nextstamp);
>>  		ehb();
>>  		if ((nextstamp - (unsigned long)read_c0_count())
>> -			> (unsigned long)LONG_MAX)
>> +			> (unsigned long)LONG_MAX) {
>> +				nextstamp = 0L;  
>>  				goto repeat;
>> +			}
>>  	}
>>  }
>>
>>
>>   
>>     
>
>