Hi Waiman,
[+Thorsten given where we are in the release cycle]
On Fri, Jan 20, 2023 at 09:17:49PM -0500, Waiman Long wrote:
The user_cpus_ptr field was originally added by commit b90ca8badbd1I'd argue it's more than just a performance regression -- the affinity
("sched: Introduce task_struct::user_cpus_ptr to track requested
affinity"). It was used only by arm64 arch due to possible asymmetric
CPU setup.
Since commit 8f9ea86fdf99 ("sched: Always preserve the user requested
cpumask"), task_struct::user_cpus_ptr is repurposed to store user
requested cpu affinity specified in the sched_setaffinity().
This results in a performance regression in an arm64 system when booted
with "allow_mismatched_32bit_el0" on the command-line. The arch code will
(amongst other things) calls force_compatible_cpus_allowed_ptr() and
relax_compatible_cpus_allowed_ptr() when exec()'ing a 32-bit or a 64-bit
task respectively. Now a call to relax_compatible_cpus_allowed_ptr()
will always result in a __sched_setaffinity() call whether there is a
previous force_compatible_cpus_allowed_ptr() call or not.
masks are set incorrectly, which is a user visible thing
(i.e. sched_getaffinity() gives unexpected values).