Re: [PATCH] pipe_read: don't wake up the writer if the pipe is still full

["Sapkal, Swapnil" <swapnil.sapkal@amd.com>]

Hi Oleg,


On 2/25/2025 5:27 PM, Oleg Nesterov wrote:
> On 02/24, Oleg Nesterov wrote:
>>
>> Just in case, did you use
>>
>> 	https://git.kernel.org/pub/scm/utils/rt-tests/rt-tests.git/tree/src/hackbench/hackbench.c
>>
>> ?
> 
> Or did you use another version?
> 

I am running hackbench using lkp-tests which downloads hackbench source 
from same rt-tests with version 2.8.

https://github.com/intel/lkp-tests.git
https://www.kernel.org/pub/linux/utils/rt-tests/rt-tests-2.8.tar.gz

> Exactly what parameters did you use?
> 

Exact command with parameters is

	/usr/bin/hackbench -g 16 -f 20 --threads --pipe -l 100000 -s 100

> If possible, please reproduce the hang again. How many threads/processes
> sleeping in pipe_read() or pipe_write() do you see? (you can look at
> /proc/$pid/stack).
> 

In the latest hang, I saw 37 threads sleeping out of which 20 were 
sleeping in pipe_read() and 17 in pipe_write().

Main hackbench thread (which spawns the readers and writers) has the 
following stack trace:

[<0>] futex_wait_queue+0x6e/0x90
[<0>] __futex_wait+0x143/0x1c0
[<0>] futex_wait+0x69/0x110
[<0>] do_futex+0x147/0x1d0
[<0>] __x64_sys_futex+0x7c/0x1e0
[<0>] x64_sys_call+0x207a/0x2140
[<0>] do_syscall_64+0x6f/0x110
[<0>] entry_SYSCALL_64_after_hwframe+0x76/0x7e

The readers have the following pipe_read stack trace:

[<0>] pipe_read+0x338/0x460
[<0>] vfs_read+0x308/0x350
[<0>] ksys_read+0xcc/0xe0
[<0>] __x64_sys_read+0x1d/0x30
[<0>] x64_sys_call+0x1b89/0x2140
[<0>] do_syscall_64+0x6f/0x110
[<0>] entry_SYSCALL_64_after_hwframe+0x76/0x7e

The writers have the following pipe_write stack trace:

[<0>] pipe_write+0x370/0x630
[<0>] vfs_write+0x378/0x420
[<0>] ksys_write+0xcc/0xe0
[<0>] __x64_sys_write+0x1d/0x30
[<0>] x64_sys_call+0x16b3/0x2140
[<0>] do_syscall_64+0x6f/0x110
[<0>] entry_SYSCALL_64_after_hwframe+0x76/0x7e

> Please pick one sleeping writer, and do
> 
> 	$ strace -p pidof_that_write
> 
> this should wake this writer up. If a missed wakeup is the only problem,
> hackbench should continue.
> 

I tried waking one of the writer and the benchmark progressed and 
completed successfully.

> The more info you can provide the better ;)
> 
> Oleg.
> 
--
Thanks and Regards,
Swapnil