<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> </head> <body text="#000000" bgcolor="#FFFFFF"> On 11/29/2017 10:19 PM, Sagar Arun Kamble wrote: <blockquote type="cite" cite="mid:48416fd2-2ea1-783c-2c53-ff9da1772ff3@intel.com"> <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> On 11/30/2017 8:34 AM, John Harrison wrote: <blockquote type="cite" cite="mid:248fd6fd-daf6-d63c-03b8-217763d6f34b@Intel.com"> On 11/24/2017 6:12 AM, Chris Wilson wrote: <blockquote type="cite" cite="mid:151153275030.23310.17279243978614545708@mail.alporthouse.com"> <pre wrap="">Quoting Michał Winiarski (2017-11-24 12:37:56) </pre> <blockquote type="cite"> <pre wrap="">Since we see the effects for GuC preeption, let's gather some evidence. (SKL) intel_guc_send_mmio latency: 100 rounds of gem_exec_latency --r '*-preemption' drm-tip: usecs : count distribution 0 -> 1 : 0 | | 2 -> 3 : 0 | | 4 -> 7 : 0 | | 8 -> 15 : 44 | | 16 -> 31 : 1088 | | 32 -> 63 : 832 | | 64 -> 127 : 0 | | 128 -> 255 : 0 | | 256 -> 511 : 12 | | 512 -> 1023 : 0 | | 1024 -> 2047 : 29899 |********* | 2048 -> 4095 : 131033 |****************************************| </pre> </blockquote> <pre wrap="">Such pretty graphs. Reminds me of the bpf hist output, I wonder if we could create a tracepoint/kprobe that would output a histogram for each waiter (filterable ofc). Benefit? Just thinking of tuning the spin/sleep, in which case overall metrics are best (intel_eait_for_register needs to be optimised for the typical case). I am wondering if we could tune the spin period down to 5us, 2us? And then have the 10us sleep. We would also need a typical workload to run, it's profile-guided optimisation after all. Hmm. -Chris </pre> </blockquote> It took me a while to get back to this but I've now had chance to run with this exponential backoff scheme on the original system that showed the problem. It was a slightly messy back port due to the customer tree being much older than current nightly. I'm pretty sure I got it correct though. However, I'm not sure what the recommendation is for the two timeout values. Using the default of '10, 10' in the patch, I still get lots of very long delays. </blockquote> Recommended setting currently is Wmin=10, Wmax=10 for wait_for_us and Wmin=10, Wmax=1000 for wait_for. Exponential backoff is more helpful inside wait_for if wait_for_us prior to wait_for is smaller. Setting Wmax less than Wmin is effectively changing the backoff strategy to just linear waits of Wmin. <blockquote type="cite" cite="mid:248fd6fd-daf6-d63c-03b8-217763d6f34b@Intel.com">I have to up the Wmin value to at least 140 to get a stall free result. Which is plausible given that the big spike in the results of any fast version is at 110-150us. Also of note is that a Wmin between 10 and 110 actually makes things worse. Changing Wmax has no effect. In the following table, 'original' is the original driver before any changes and 'retry loop' is the version using the first workaround of just running the busy poll wait in a 10x loop. The other columns are using the backoff patch with the given Wmin/Wmax values. Note that the times are bucketed to 10us up to 500us and then in 500us lumps thereafter. The value listed is the lower limit, i.e. there were no times of <10us measured. Each case was run for 1000 samples. </blockquote> Below setting like in current nightly will suit this workload and as you have found this will also likely complete most waits in <150us. If many samples had been beyond 160us and less than 300us we might have been needed to change Wmin to may be 15 or 20 to ensure the exponential rise caps around 300us. wait_for_us(10, 10) wait_for() #define wait_for _wait_for(10, 1000) </blockquote> But as shown in the table, a setting of 10/10 does not work well for this workload. The best results possible are a large spike of waits in the 120-130us bucket with a small tail out to 150us. Whereas, the 10/10 setting produces a spike from 150-170us with the tail extending to 240us and an appreciable number of samples stretching all the way out to the 1-10ms range. A regular delay of multiple milliseconds is not acceptable when this path is supposed to be a low latency pre-emption to switch to some super high priority time critical task. And as noted, I did try a bunch of different settings for Wmax but nothing seemed to make much of a difference. E.g. 10/10 vs 10/1000 produced pretty much identical results. Hence it didn't seem worth including those in the table. <blockquote type="cite" cite="mid:48416fd2-2ea1-783c-2c53-ff9da1772ff3@intel.com"> <blockquote type="cite" cite="mid:248fd6fd-daf6-d63c-03b8-217763d6f34b@Intel.com"> <tt><tt> Time Original 10/10 50/10 100/10 110/10 130/10 140/10 RetryLoop 10us: 2 2 2 2 2 2 2 2 30us: 1 1 1 1 1 50us: 1 70us: 14 63 56 64 63 61 80us: 8 41 52 44 46 41 90us: 6 24 10 28 12 17 100us: 2 4 20 16 17 17 22 110us: 13 21 14 13 11 120us: 6 366 633 636 660 650 130us: 2 2 46 125 95 86 95 140us: 3 2 16 18 32 46 48 150us: 210 3 12 13 37 32 31 160us: 322 1 18 10 14 12 17 170us: 157 4 5 5 3 5 2 180us: 62 11 3 1 2 1 1 190us: 32 212 1 1 2 200us: 27 266 1 1 210us: 16 181 1 220us: 16 51 1 230us: 10 43 4 240us: 12 22 62 1 250us: 4 12 112 3 260us: 3 13 73 8 270us: 5 12 12 8 2 280us: 4 7 12 5 1 290us: 9 4 300us: 1 3 9 1 1 310us: 2 3 5 1 1 320us: 1 4 2 3 330us: 1 5 1 340us: 1 2 1 350us: 2 1 360us: 2 1 370us: 2 2 380us: 1 390us: 2 1 2 1 410us: 1 420us: 3 430us: 2 2 1 440us: 2 1 450us: 4 460us: 3 1 470us: 3 1 480us: 2 2 490us: 1 500us: 19 13 17 1000us: 249 22 30 11 1500us: 393 4 4 2 1 2000us: 132 7 8 8 2 1 1 2500us: 63 4 4 6 1 1 1 3000us: 59 9 7 6 1 3500us: 34 2 1 1 4000us: 17 9 4 1 4500us: 8 2 1 1 5000us: 7 1 2 5500us: 7 2 1 6000us: 4 2 1 1 6500us: 3 1 7000us: 6 2 1 7500us: 4 1 1 8000us: 5 1 8500us: 1 1 9000us: 2 9500us: 2 1 >10000us: 3 1 </tt></tt> John. <fieldset class="mimeAttachmentHeader"></fieldset> <pre wrap="">_______________________________________________ Intel-gfx mailing list <a class="moz-txt-link-abbreviated" href="mailto:Intel-gfx@lists.freedesktop.org" moz-do-not-send="true">Intel-gfx@lists.freedesktop.org</a> <a class="moz-txt-link-freetext" href="https://lists.freedesktop.org/mailman/listinfo/intel-gfx" moz-do-not-send="true">https://lists.freedesktop.org/mailman/listinfo/intel-gfx</a> </pre> </blockquote> </blockquote> </body> </html>