Re: [PATCH v4] drm/i915: Move execlists irq handler to a bottom half

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 04/04/16 12:27, Chris Wilson wrote:
> On Mon, Apr 04, 2016 at 12:11:56PM +0100, Tvrtko Ursulin wrote:
>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>
>> 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.
>>
>> When a interrupt "cliff" is reached, which was >~320k irq/s on
>> my machine, the whole system goes into a terrible state of the
>> above described multi-second lockups.
>>
>> By moving the GT interrupt handling to a tasklet in a most
>> simple way, the problem above disappears completely.
>>
>> Testing the effect on sytem-wide latencies using
>> igt/gem_syslatency shows the following before this patch:
>>
>> 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
>>
>> This shows that the unrelated process is experiencing huge
>> delays in its wake-up latency. After the patch the results
>> look like this:
>>
>> 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
>>
>> Showing a huge improvement in the unrelated process wake-up
>> latency. It also shows an approximate halving in the number
>> of total empty batches submitted during the test. This may
>> not be worrying since the test puts the driver under
>> a very unrealistic load with ncpu threads doing empty batch
>> submission to all GPU engines each.
>>
>> Another benefit compared to the hard-irq handling is that now
>> work on all engines can be dispatched in parallel since we can
>> have up to number of CPUs active tasklets. (While previously
>> a single hard-irq would serially dispatch on one engine after
>> another.)
>>
>> More interesting scenario with regards to throughput is
>> "gem_latency -n 100" which  shows 25% better throughput and
>> CPU usage, and 14% better dispatch latencies.
>>
>> I did not find any gains or regressions with Synmark2 or
>> GLbench under light testing. More benchmarking is certainly
>> required.
>>
>> v2:
>>     * execlists_lock should be taken as spin_lock_bh when
>>       queuing work from userspace now. (Chris Wilson)
>>     * uncore.lock must be taken with spin_lock_irq when
>>       submitting requests since that now runs from either
>>       softirq or process context.
>>
>> v3:
>>     * Expanded commit message with more testing data;
>>     * converted missed locking sites to _bh;
>>     * added execlist_lock comment. (Chris Wilson)
>>
>> v4:
>>     * Mention dispatch parallelism in commit. (Chris Wilson)
>>     * Do not hold uncore.lock over MMIO reads since the block
>>       is already serialised per-engine via the tasklet itself.
>>       (Chris Wilson)
>>     * intel_lrc_irq_handler should be static. (Chris Wilson)
>>     * Cancel/sync the tasklet on GPU reset. (Chris Wilson)
>>     * Document and WARN that tasklet cannot be active/pending
>>       on engine cleanup. (Chris Wilson/Imre Deak)
>>
>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>> Cc: Chris Wilson <chris@chris-wilson.co.uk>
>> Cc: Imre Deak <imre.deak@intel.com>
>> Testcase: igt/gem_exec_nop/all
>> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94350
>> Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
>
> Didn't spot anything else in testing over the last week. There are a
> number of follow up improvements we can make to intel_lrc_irq_handler()
> to halve its execution time (minor improvement to execlist throughput,
> major improvement to syslatency) which focus on streamlining the
> register reads and context-unqueueing (but the patches I have depend on
> esoteric features like struct_mutex-less requests).

Would that be the just "drm/i915: Move releasing of the GEM request from 
free to retire/cancel" or more? The former could be pulled out from your 
pile easily even if it doesn't solve anything major on its own it is a 
nice cleanup.

Regards,

Tvrtko
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