Re: [PATCH] sched: fix timeval conversion to jiffies

[Paul Turner]

On Thu, Sep 4, 2014 at 2:30 PM, John Stultz <john.stultz@xxxxxxxxxx> wrote:
> On Thu, Sep 4, 2014 at 2:17 PM, Andrew Hunter <ahh@xxxxxxxxxx> wrote:
>> On Wed, Sep 3, 2014 at 5:06 PM, John Stultz <john.stultz@xxxxxxxxxx> wrote:
>>> Maybe with the next version of the patch, before you get into the
>>> unwinding the math, you might practically describe what is broken,
>>> then explain how its broken.
>>>
>>> My quick read here is that we're converting a timespec -> jiffies, and
>>> in doing so we round up by one jiffy.
>>>
>>> This seems actually perfectly normal, as we usually end up rounding up
>>> by a jiffy in many cases since we don't want to undershoot any
>>> timeout, and we're always allowed return later then specified.
>>
>> Well, yes, timeouts can be longer than specified, but what you said
>> technically applies just as much to code that arbitrarily multiplies
>> jiffies by 10 before returning, no? :)
>>
>> The problem isn't the rounding, it's that the rounding is _wrong_: I'm
>> fine rounding 10100usec / (1000 usec/jiffie) = 11 jiffies. The current
>> code rounds 10000usec / (1000 usec/jiffies) to 11.  I've rewritten the
>> description to make this clearer.
>
> Ok. Very much appreciated!
>
>>>> In particular, with HZ=1000, we consistently computed that 10000 usec
>>>> was 11 jiffies; the same was true for any exact multiple of
>>>> TICK_NSEC. This is obviously bad as a general rule, and caused
>>>> observable user problems with setitimer() at the very least:
>>>>
>>>> setitimer(ITIMER_PROF, &val, NULL);
>>>> setitimer(ITIMER_PROF, NULL, &val);
>>>>
>>>> would actually add a tick to val!
>>>
>>> So this looks like an issue. Since we convert and store the internal
>>> representation in jiffies, when we pull it back out, we get the
>>> rounded up value, which is larger then the timespec value originally
>>> submitted.  This is really the core issue, correct?
>>
>> For the particular user bug reported to me, yes, this was the core
>> issue: some code that stopped and restarted an itimer found the
>> interval growing by 1ms each time.  But again, it wasn't that it was
>> rounded:  if we initially passed e.g. 10500 usec and got back 11000,
>> that'd be annoying but workable, because if we then went through
>> another cycle of enabling/disabling itimer, we'd set it to 11000 usec
>> and get back 11000 again.  What we have now instead adds a full jiffie
>> _every time_.
>
> Ah, ok. This part is key to understanding the problem. Thanks for
> clarifying this.
>
> This seems to be a quite old bug.. Do you think this is needed for -stable?

Seems reasonable to me.

>
> thanks
> -john
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