On 11/30/2017 12:45 PM, John Harrison wrote:
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.
Wmin = 10us leads us to total delay of 150us in 3 loops (this might be
tight to catch most conditions)
Wmin = 25us can lead us to total delay of 175us in 3 loops
Since most conditions are likely to complete around 140us-160us, Looks
like Wmin of 25 to 30 (25,1000 or 30, 1000) will suit this workload but
since this profile driver optimization I am wondering about the optimal
Wmin point.
This wait need is very time critical. Exponential rise might not be good
strategy during higher wait times.
usleep_range might also be adding extra latency.
May be we should do this exponential backoff for waits having US >= 1000
and do periodic backoff for US<1000 with period of 50us?
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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