Re: [rfc] lru_add_drain_all() vs isolation

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Tue, 2009-09-08 at 19:06 +0900, KOSAKI Motohiro wrote:
> > On Tue, 2009-09-08 at 08:56 +0900, KOSAKI Motohiro wrote:
> > > Hi Peter,
> > > 
> > > > On Mon, 2009-09-07 at 10:17 +0200, Mike Galbraith wrote:
> > > > 
> > > > > [  774.651779] SysRq : Show Blocked State
> > > > > [  774.655770]   task                        PC stack   pid father
> > > > > [  774.655770] evolution.bin D ffff8800bc1575f0     0  7349   6459 0x00000000
> > > > > [  774.676008]  ffff8800bc3c9d68 0000000000000086 ffff8800015d9340 ffff8800bb91b780
> > > > > [  774.676008]  000000000000dd28 ffff8800bc3c9fd8 0000000000013340 0000000000013340
> > > > > [  774.676008]  00000000000000fd ffff8800015d9340 ffff8800bc1575f0 ffff8800bc157888
> > > > > [  774.676008] Call Trace:
> > > > > [  774.676008]  [<ffffffff812c4a11>] schedule_timeout+0x2d/0x20c
> > > > > [  774.676008]  [<ffffffff812c4891>] wait_for_common+0xde/0x155
> > > > > [  774.676008]  [<ffffffff8103f1cd>] ? default_wake_function+0x0/0x14
> > > > > [  774.676008]  [<ffffffff810c0e63>] ? lru_add_drain_per_cpu+0x0/0x10
> > > > > [  774.676008]  [<ffffffff810c0e63>] ? lru_add_drain_per_cpu+0x0/0x10
> > > > > [  774.676008]  [<ffffffff812c49ab>] wait_for_completion+0x1d/0x1f
> > > > > [  774.676008]  [<ffffffff8105fdf5>] flush_work+0x7f/0x93
> > > > > [  774.676008]  [<ffffffff8105f870>] ? wq_barrier_func+0x0/0x14
> > > > > [  774.676008]  [<ffffffff81060109>] schedule_on_each_cpu+0xb4/0xed
> > > > > [  774.676008]  [<ffffffff810c0c78>] lru_add_drain_all+0x15/0x17
> > > > > [  774.676008]  [<ffffffff810d1dbd>] sys_mlock+0x2e/0xde
> > > > > [  774.676008]  [<ffffffff8100bc1b>] system_call_fastpath+0x16/0x1b
> > > > 
> > > > FWIW, something like the below (prone to explode since its utterly
> > > > untested) should (mostly) fix that one case. Something similar needs to
> > > > be done for pretty much all machine wide workqueue thingies, possibly
> > > > also flush_workqueue().
> > > 
> > > Can you please explain reproduce way and problem detail?
> > > 
> > > AFAIK, mlock() call lru_add_drain_all() _before_ grab semaphoe. Then,
> > > it doesn't cause any deadlock.
> > 
> > Suppose you have 2 cpus, cpu1 is busy doing a SCHED_FIFO-99 while(1),
> > cpu0 does mlock()->lru_add_drain_all(), which does
> > schedule_on_each_cpu(), which then waits for all cpus to complete the
> > work. Except that cpu1, which is busy with the RT task, will never run
> > keventd until the RT load goes away.
> > 
> > This is not so much an actual deadlock as a serious starvation case.
> 
> This seems flush_work vs RT-thread problem, not only lru_add_drain_all().
> Why other workqueue flusher doesn't affect this issue?

flush_work() will only flush workqueues on which work has been enqueued
as Oleg pointed out.

The problem is with lru_add_drain_all() enqueueing work on all
workqueues.

There is nothing that makes lru_add_drain_all() the only such site, its
the one Mike posted to me, and my patch was a way to deal with that.

I also explained that its not only RT related in that the HPC folks also
want to avoid unneeded work -- for them its not starvation but a
performance issue.

In generic we should avoid doing work when there is no work to be done.