On 08/06/2018 04:14 AM, Christian König wrote:
Am 03.08.2018 um 20:41 schrieb Andrey Grodzovsky:
On 08/06/2018 08:44 AM, Christian König wrote:
Am 03.08.2018 um 16:54 schrieb Andrey Grodzovsky:
[SNIP]
>
> Second of all, even after we removed the entity from rq in
> drm_sched_entity_flush to terminate any subsequent submissions
>
> to the queue the other thread doing push job can just
acquire again a
> run queue
>
> from drm_sched_entity_get_free_sched and continue submission
Hi Christian
That is actually desired.
When another process is now using the entity to submit jobs
adding it
back to the rq is actually the right thing to do cause the
entity is
still in use.
Yes, no problem if it's another process. But what about another
thread from same process ? Is it a possible use case that 2
threads from same process submit to same entity concurrently ? If
so and we specifically kill one, the other will not stop event if
we want him to because current code makes him just require a rq
for him self.
Well you can't kill a single thread of a process (you can only
interrupt it), a SIGKILL will always kill the whole process.
Is the following scenario possible and acceptable ?
2 threads from same process working on same queue where thread A
currently in drm_sched_entity_flush->wait_event_timeout (the
process getting shut down because of SIGKILL sent by user)
while thread B still inside drm_sched_entity_push_job before 'if
(reschedule)'. 'A' stopped waiting because queue became empty and
then removes the entity queue from scheduler's run queue while
B goes inside 'reschedule' because it evaluates to true ('first' is
true and all the rest of the conditions), acquires new rq, and
later adds it back to scheduler (different one maybe) and keeps
submitting jobs as much as he likes and then can be stack for up to
'timeout' time in his drm_sched_entity_flush waiting for them.
I'm not 100% sure but I don't think that can happen.
See flushing the fd is done while dropping the fd, which happens
only after all threads of the process in question are killed.
Yea, this FDs handling is indeed a lot of gray area for me but as far
as I remember flushing is done per each thread when exits (possibly
due to a signal).
Now signals interception and processing (as a result of which .flush
will get called if SIGKILL received) is done in some points amongst
which is when returning from IOCTL.
So if first thread was at the very end of the CS ioctl when SIGKILL
was received while the other one at the beginning then I think we
might see something like the scenario above.
SIGKILL isn't processed as long as any thread of the application is
still inside the kernel. That's why we have wait_event_killable().
Can you tell me where is this happening ? What i see is in the code is that
do_group_exit calls zap_other_threads which just adds SIGKILL to signal
sets of other threads in group and sends a wake up.
Then do_exit will close all FDs for current thread and so .flush will be
called, when last thread drops it's refcount for the FD .release will be
called.
Andrey
So I don't think that the scenario above is possible, but I'm really
not 100% sure either.
Christian.
Andrey
Otherwise the flushing wouldn't make to much sense. In other words
imagine an application where a thread does a write() on a fd which
is killed.
The idea of the flush is to preserve the data and that won't work if
that isn't correctly ordered.
My understanding was that introduction of entity->last is to force
immediate termination job submissions by any thread from the
terminating process.
We could consider reordering that once more. Going to play out all
scenarios in my head over the weekend :)
Christian.
Andrey
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