On Mon, Apr 04, 2016 at 12:11:56PM +0100, 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. > > 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@xxxxxxxxx> > Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> > Cc: Imre Deak <imre.deak@xxxxxxxxx> > Testcase: igt/gem_exec_nop/all > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94350 > Reviewed-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> 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). Lgtm, -Chris -- Chris Wilson, Intel Open Source Technology Centre _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/intel-gfx
