Re: [Qemu-block] [PATCH for-2.12 v2] qemu-iotests: update 185 output

[Max Reitz]

On 2018-04-10 10:11, Stefan Hajnoczi wrote:
> On Wed, Apr 04, 2018 at 06:16:12PM +0200, Max Reitz wrote:
>> On 2018-04-04 17:01, Stefan Hajnoczi wrote:
>>  === Start mirror job and exit qemu ===
>>
>> This seems to be independent of whether there is actually data on
>> TEST_IMG (the commit source), so something doesn't seem quite right with
>> the block job throttling here...?
> 
> I've been playing around with this test failure.  Let's leave it broken
> (!) in QEMU 2.12 because this test has uncovered a block job ratelimit
> issue that's not a regression.  The fix shouldn't be rushed.

OK for me.

> block/mirror.c calculates delay_ns but then discards it:
> 
>   static void coroutine_fn mirror_run(void *opaque)
>   {
>       ...
> 
>       for (;;) {
>           ...delay_ns is calculated based on job speed...
> 
>           ret = 0;
>           trace_mirror_before_sleep(s, cnt, s->synced, delay_ns);
>           if (block_job_is_cancelled(&s->common) && s->common.force) {
>               break;
>           } else if (!should_complete) {
>               delay_ns = (s->in_flight == 0 && cnt == 0 ? SLICE_TIME : 0);
>               ^--- we discard it!
>               block_job_sleep_ns(&s->common, delay_ns);
>           }
>           s->last_pause_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
>       }
> 
>       ...
>   }

Uh, nice.

> This is why the mirror-based tests are completing even though we'd
> expect them to only reach the "next" iteration due to the job speed.
> 
> Increasing the 4 MB write operation in the test to >16 MB (the mirror
> buffer size) doesn't solve the problem because the next QMP command will
> race with the job's 0 ns sleep.  It would just make the test unreliable.
> 
> I'll work on the following for QEMU 2.13:
> 
> 1. Avoid spurious block_job_enter() from block_job_drain().  This
>    eliminates the extra block job iteration that changed the output in
>    the first place.
> 
> 2. Honor the job speed in block/mirror.c.
> 
> The end result will be that the test output will not require changes.

Thanks!

Max

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