<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> </head> <body text="#000000" bgcolor="#FFFFFF"> 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. 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. <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. </body> </html>