On 11/29/2017 11:55 PM, Sagar Arun Kamble wrote:
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?
The results I am seeing do not correspond. First of, it seems I get
different results depending upon the context. That is in the context
of the pre-emption GuC send action command I get the results
previously posted. If I just run usleep_range(x, y) in loop 1000
times from the context of dumping a debugfs file, I get something
very different. Basically, the minimum sleep time is 110-120us
irrespective of the values of X and Y. Pushing X and Y beyond 120
seems to make it complete in Y+10-20us. E.g. u_r(100,200) completes
in 210-230us for 80% of samples. On the other hand, I don't get
anywhere near so many samples in the millisecond range as when
called in the send action code path.
However, it sounds like the underlying issue might actually be a
back-port merge problem. The customer tree in question is actually a
combination of a 4.1 base kernel with a 4.11 DRM dropped on top. As
noted in a separate thread, this tree also has a problem with the
mutex_lock() call stalling even when the mutex is very definitely
not acquired (using mutex_trylock() eliminates the stall
completely). Apparently the back port process did hit a bunch of
conflicts in the base kernel's scheduling code. So there is a strong
possibility that all the issues we are seeing in that tree are an
artifact of a merge issue.
So I think it is probably safe to ignore the results I am seeing in
terms of what the best upstream solution should be.
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.
_______________________________________________
Intel-gfx mailing list
Intel-gfx@xxxxxxxxxxxxxxxxxxxxx
https://lists.freedesktop.org/mailman/listinfo/intel-gfx
|
