On 11/29/2017 10:19 PM, Sagar Arun Kamble wrote:
On 11/30/2017 8:34 AM, John Harrison wrote:
On 11/24/2017 6:12 AM, Chris Wilson wrote:
Quoting Michał Winiarski (2017-11-24 12:37:56)
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 |****************************************|
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
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.
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.
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.
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)
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.
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
John.
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