Hi David,
I've been experiencing a performance regression when running a parallel
compilation (eg, make -j72) on recent kernels.
I bisected it to this commit:
commit b667b867344301e24f21d4a4c844675ff61d89e1
Author: David Howells <dhowells@xxxxxxxxxx>
Date: Tue Sep 24 16:09:04 2019 +0100
pipe: Advance tail pointer inside of wait spinlock in pipe_read()
Advance the pipe ring tail pointer inside of wait spinlock in pipe_read()
so that the pipe can be written into with kernel notifications from
contexts where pipe->mutex cannot be taken.
Prior to this commit I got 70% or so thread saturation of cc1 and after
it rarely gets above 15% and would often drop down to 1-2 threads. It
doesn't look like a clean revert either. Looking at upstream, it seems
that some later code changed the wakeup. I'm not really sure how this
all fits into parallelized make.
Best
Jon
