Re: [Xen-devel] [PATCH -v3 1/1] ratelimit: Implement rate limit for credit2 scheduler Rate limit assures that a vcpu will execute for a minimum amount of time before being put at the back of a queue or being preempted by higher priority thread.

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 26/07/16 17:21, Dario Faggioli wrote:
> On Mon, 2016-07-25 at 17:12 +0100, Anshul Makkar wrote:
>> It introduces context-switch rate-limiting. The patch enables the VM
>> to batch
>> its work and prevents the system from spending most of its time in
>> context
>> switches because of a VM that is waking/sleeping at high rate.
>>
>> ratelimit can be disabled by setting it to 0.
>>
> Thanks Anshul. I've looked at the patch, and it seemed all right to me.
> 
> I decided to build and test it, and I've seem something that I found
> weird.
> 
> But first of all, one thing that I'm sorry I'm mentioning now
> (especially considering it was quite evident! :-/).
> 
> The subject, which will become the "title" of the commit, is way too
> long. That must be a very concise headline of what the patch is about,
> and should also have tags, specifying what areas of the codebase are
> affected. So what do you think of this:
> 
>   xen: credit2: implement context switch rate-limiting.

It looks like it's just missing a carrage return -- I could fix that up
on check-in.

> 
> On a more technical side, I think that...
> 
>> @@ -2116,9 +2147,22 @@ csched2_runtime(const struct scheduler *ops,
>> int cpu, struct csched2_vcpu *snext
>>       * 1) Run until snext's credit will be 0
>>       * 2) But if someone is waiting, run until snext's credit is
>> equal
>>       * to his
>> -     * 3) But never run longer than MAX_TIMER or shorter than
>> MIN_TIMER.
>> +     * 3) But never run longer than MAX_TIMER or shorter than
>> MIN_TIMER or
>> +     * the ratelimit time.
>>       */
>>  
>> +    /* Calculate mintime */
>> +    min_time = CSCHED2_MIN_TIMER;
>> +    if ( prv->ratelimit_us )
>> +    {
>> +        s_time_t ratelimit_min = prv->ratelimit_us;
>>
> ... this should be:
> 
>  s_time_t ratelimit_min = MICROSECS(prv->ratelimit_us);
> 
> I realized that by looking at traces and seeing entries for which
> CSCHED2_MIN_TIMER was being returned, even if I had set
> sched_ratelimit_us to a value greater than that.

Yes, Dario is correct here.

There's also a small typo in one of the comments ("onw" instead of "own").

With all those changed:

Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>

If Anshul and Dario are happy, I can fix all those thing up on commit.

-George

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