On 23/03/16 11:31, Chris Wilson wrote:
On Wed, Mar 23, 2016 at 10:08:46AM +0000, Tvrtko Ursulin wrote:
On 23/03/16 09:14, Chris Wilson wrote:
On Wed, Mar 23, 2016 at 10:07:35AM +0100, Daniel Vetter wrote:
On Tue, Mar 22, 2016 at 05:30:04PM +0000, Tvrtko Ursulin wrote:
From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Doing a lot of work in the interrupt handler introduces huge
latencies to the system as a whole.
Most dramatic effect can be seen by running an all engine
stress test like igt/gem_exec_nop/all where, when the kernel
config is lean enough, the whole system can be brought into
multi-second periods of complete non-interactivty. That can
look for example like this:
NMI watchdog: BUG: soft lockup - CPU#0 stuck for 23s! [kworker/u8:3:143]
Modules linked in: [redacted for brevity]
CPU: 0 PID: 143 Comm: kworker/u8:3 Tainted: G U L 4.5.0-160321+ #183
Hardware name: Intel Corporation Broadwell Client platform/WhiteTip Mountain 1
Workqueue: i915 gen6_pm_rps_work [i915]
task: ffff8800aae88000 ti: ffff8800aae90000 task.ti: ffff8800aae90000
RIP: 0010:[<ffffffff8104a3c2>] [<ffffffff8104a3c2>] __do_softirq+0x72/0x1d0
RSP: 0000:ffff88014f403f38 EFLAGS: 00000206
RAX: ffff8800aae94000 RBX: 0000000000000000 RCX: 00000000000006e0
RDX: 0000000000000020 RSI: 0000000004208060 RDI: 0000000000215d80
RBP: ffff88014f403f80 R08: 0000000b1b42c180 R09: 0000000000000022
R10: 0000000000000004 R11: 00000000ffffffff R12: 000000000000a030
R13: 0000000000000082 R14: ffff8800aa4d0080 R15: 0000000000000082
FS: 0000000000000000(0000) GS:ffff88014f400000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fa53b90c000 CR3: 0000000001a0a000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
042080601b33869f ffff8800aae94000 00000000fffc2678 ffff88010000000a
0000000000000000 000000000000a030 0000000000005302 ffff8800aa4d0080
0000000000000206 ffff88014f403f90 ffffffff8104a716 ffff88014f403fa8
Call Trace:
<IRQ>
[<ffffffff8104a716>] irq_exit+0x86/0x90
[<ffffffff81031e7d>] smp_apic_timer_interrupt+0x3d/0x50
[<ffffffff814f3eac>] apic_timer_interrupt+0x7c/0x90
<EOI>
[<ffffffffa01c5b40>] ? gen8_write64+0x1a0/0x1a0 [i915]
[<ffffffff814f2b39>] ? _raw_spin_unlock_irqrestore+0x9/0x20
[<ffffffffa01c5c44>] gen8_write32+0x104/0x1a0 [i915]
[<ffffffff8132c6a2>] ? n_tty_receive_buf_common+0x372/0xae0
[<ffffffffa017cc9e>] gen6_set_rps_thresholds+0x1be/0x330 [i915]
[<ffffffffa017eaf0>] gen6_set_rps+0x70/0x200 [i915]
[<ffffffffa0185375>] intel_set_rps+0x25/0x30 [i915]
[<ffffffffa01768fd>] gen6_pm_rps_work+0x10d/0x2e0 [i915]
[<ffffffff81063852>] ? finish_task_switch+0x72/0x1c0
[<ffffffff8105ab29>] process_one_work+0x139/0x350
[<ffffffff8105b186>] worker_thread+0x126/0x490
[<ffffffff8105b060>] ? rescuer_thread+0x320/0x320
[<ffffffff8105fa64>] kthread+0xc4/0xe0
[<ffffffff8105f9a0>] ? kthread_create_on_node+0x170/0x170
[<ffffffff814f351f>] ret_from_fork+0x3f/0x70
[<ffffffff8105f9a0>] ? kthread_create_on_node+0x170/0x170
I could not explain, or find a code path, which would explain
a +20 second lockup, but from some instrumentation it was
apparent the interrupts off proportion of time was between
10-25% under heavy load which is quite bad.
By moving the GT interrupt handling to a tasklet in a most
simple way, the problem above disappears completely.
Also, gem_latency -n 100 shows 25% better throughput and CPU
usage, and 14% better latencies.
Forgot gem_syslatency, since that does reflect UX under load really
startlingly well.
gem_syslatency, before:
gem_syslatency: cycles=1532739, latency mean=416531.829us max=2499237us
gem_syslatency: cycles=1839434, latency mean=1458099.157us max=4998944us
gem_syslatency: cycles=1432570, latency mean=2688.451us max=1201185us
gem_syslatency: cycles=1533543, latency mean=416520.499us max=2498886us
with tasklet:
gem_syslatency: cycles=808907, latency mean=53.133us max=1640us
gem_syslatency: cycles=862154, latency mean=62.778us max=2117us
gem_syslatency: cycles=856039, latency mean=58.079us max=2123us
gem_syslatency: cycles=841683, latency mean=56.914us max=1667us
Is this smaller throughput and better latency?
Yeah. I wasn't expecting the smaller throughput, but the impact on other
users is massive. You should be able to feel the difference if you try
to use the machine whilst gem_syslatency or gem_exec_nop is running, a
delay of up to 2s in responding to human input can be annoying!
Yes, impact is easily felt.
gem_exec_nop/all has a huge improvement also, if we ignore the fact
it locks up the system with the curret irq handler on full tilt,
when I limit the max GPU frequency a bit it can avoid that problem
but tasklets make it twice as fast here.
Yes, with threaded submission can then concurrently submit requests to
multiple rings. I take it you have a 2-processor machine? We should
ideally see linear scaling upto min(num_engines, nproc-1) if we assume
that one cpu is enough to sustain gem_execbuf() ioctls.
2C/4T correct.
I did not find any gains or regressions with Synmark2 or
GLbench under light testing. More benchmarking is certainly
required.
Bugzilla?
You think it is OK to continue sharing your one,
https://bugs.freedesktop.org/show_bug.cgi?id=93467?
Yes, it fixes the same freeze (and we've removed the loop from chv irq
so there really shouldn't be any others left!)
I don't see that has been merged. Is it all ready CI wise so we could?
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
I thought tasklets are considered unpopular nowadays? They still steal cpu
Did not know, last (and first) time I've used them was ~15 years
ago. :) You got any links to read about it? Since (see below) I am
not sure they "steal" CPU time.
time, just have the benefit of not also disabling hard interrupts. There
should be mitigation though to offload these softinterrupts to threads.
Have you tried to create a threaded interrupt thread just for these pins
instead? A bit of boilerplate, but not much using the genirq stuff iirc.
Ah, you haven't been reading patches. Yes, there's been a patch to fix
the hardlockup using kthreads for a few months. Tvrtko is trying to move
this forward since he too has found a way of locking up his machine
using execlist under load.
Correct.
So far kthreads seems to have a slight edge in the benchmarks, or rather
using tasklet I have some very wild results on Braswell. Using tasklets
the CPU time is accounted to the process (i.e. whoever was running at
the time of the irq, typically the benchmark), using kthread we have
I thought they run from ksoftirqd so the CPU time is accounted
against it. And looking at top, that even seems what actually
happens.
Not for me. :| Though I'm using simple CPU time accounting.
I suppose it must be this one you don't have then:
config IRQ_TIME_ACCOUNTING
bool "Fine granularity task level IRQ time accounting"
depends on HAVE_IRQ_TIME_ACCOUNTING && !NO_HZ_FULL
help
Select this option to enable fine granularity task irq time
accounting. This is done by reading a timestamp on each
transitions between softirq and hardirq state, so there can
be a small performance impact.
independent entries in the process table/top (which is quite nice to see
just how much time is been eaten up by the context-switches).
Benchmarks still progessing, haven't yet got on to the latency
measureemnts....
My tasklets hack required surprisingly little code change, at least
if there are not some missed corner cases to handle, but I don't
mind your threads either.
Yes, though when moving to kthreads I dropped the requirement for
spin_lock_irq(engine->execlists_lock) and so there is a large amount of
fluff in changing those callsites to spin_lock(). (For tasklet, we could
argue that requirement is now changed to spin_lock_bh()...) The real meat
Ooops yes, _bh variant is the correct one. I wonder if that would
further improve things. Will try.
of the change is that with kthreads we have to worry about doing the
scheduling() ourselves, and that impacts upon the forcewake dance so
certainly more complex than tasklets! I liked how simple this patch is
and so far it looks as good as making our own kthread. The biggest
difference really is just who gets the CPU time!
Note that in that respect it is then no worse than the current situation
wrt CPU time accounting.
Regards,
Tvrtko
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