Parallel compilation performance regression

[Derrick, Jonathan]

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