On Fri, Dec 17, 2021 at 1:43 PM Daniel Vacek [off-list ref] wrote:

Hello,

This is perfectly correct and expected result. Further explanation
follows down below.

On Wed, Dec 15, 2021 at 8:41 PM [off-list ref] wrote:

quoted

December 15, 2021 1:27 AM, "Song Liu" [off-list ref] wrote:

quoted

On Mon, Dec 13, 2021 at 7:17 PM Yajun Deng [off-list ref] wrote:

quoted

We found an abnormally high latency when executing modprobe raid6_pq, the
latency is greater than 1.2s when CONFIG_PREEMPT_VOLUNTARY=y, greater than
67ms when CONFIG_PREEMPT=y, and greater than 16ms when CONFIG_PREEMPT_RT=y.
This is caused by disable the preemption, this time is too long and
unreasonable. We just need to disable migration. so used migrate_disable()/
migrate_enable() instead of preempt_disable()/preempt_enable(). This is
beneficial for CONFIG_PREEMPT=y or CONFIG_PREEMPT_RT=y, but no effect for
CONFIG_PREEMPT_VOLUNTARY=y.

Fixes: fe5cbc6e06c7 ("md/raid6 algorithms: delta syndrome functions")
Signed-off-by: Yajun Deng <redacted>

We measure the speed of different RAID algorithms.If we don't disable
preempt, the result may be inaccurate, right? IIUC, we only disable preempt
for 16 jiffies. Why do we see 1.2 second delay?

Here are the command of my test：
Execute "sudo cyclictest -S -p 95 -d 0 -i 1000 -D 24h -m" in one terminal and "sudo modprobe raid6_pq" in the other terminal.

Here are the results of my test：
CONFIG_PREEMPT_VOLUNTARY=y，CONFIG_HZ_250=y
T:17 ( 3109) P:95 I:1000 C:   7255 Min:      1 Act:    2 Avg:  170 Max: 1220832

No preemption is expected with this config so the CPU is busy and
occupied for full duration of all the benchmarks.

quoted

CONFIG_PREEMPT=y,CONFIG_HZ_250=y
T: 2 ( 2462) P:95 I:1000 C:  49024 Min:      1 Act:    2 Avg:    4 Max:   67888

16 jiffies with 250 Hz equals to 64 ms. So again, this result is
perfectly expected. The benchmarks can be preempted in between the
individual measurements with preemptible kernel configuration.

quoted

CONFIG_PREEMPT_RT=y,CONFIG_HZ_1000=y
T: 6 ( 2561) P:95 I:1000 C:  25284 Min:      3 Act:    3 Avg:    4 Max:   16593

16 jiffies by 1000 Hz is 16 ms. Ditto. So far so good.

Thanks for the explanation. I misread/misunderstood some of these results.
I dropped the patch from md-next.

Song