Re: [RFC PATCH 2/4] swap: apply new local_schedule_work_on() interface

[Marcelo Tosatti <mtosatti@redhat.com>]

On Sat, Jul 29, 2023 at 05:37:33AM -0300, Leonardo Bras wrote:
> Make use of the new local_*lock_n*() and local_schedule_work_on() interface
> to improve performance & latency on PREEMTP_RT kernels.
> 
> For functions that may be scheduled in a different cpu, replace
> local_*lock*() by local_lock_n*(), and replace schedule_work_on() by
> local_schedule_work_on(). The same happens for flush_work() and
> local_flush_work().
> 
> This should bring no relevant performance impact on non-RT kernels:
> For functions that may be scheduled in a different cpu, the local_*lock's
> this_cpu_ptr() becomes a per_cpu_ptr(smp_processor_id()).
> 
> Signed-off-by: Leonardo Bras <leobras@redhat.com>
> ---
>  mm/swap.c | 18 +++++++++---------
>  1 file changed, 9 insertions(+), 9 deletions(-)

Leo,

I think the interruptions should rather be removed for both
CONFIG_PREEMPT_RT AND !CONFIG_PREEMPT_RT.

The impact of grabbing locks must be properly analyzed and not
"rejected blindly".

Example:

commit 01b44456a7aa7c3b24fa9db7d1714b208b8ef3d8
Author: Mel Gorman <mgorman@techsingularity.net>
Date:   Fri Jun 24 13:54:23 2022 +0100

    mm/page_alloc: replace local_lock with normal spinlock
    
    struct per_cpu_pages is no longer strictly local as PCP lists can be
    drained remotely using a lock for protection.  While the use of local_lock
    works, it goes against the intent of local_lock which is for "pure CPU
    local concurrency control mechanisms and not suited for inter-CPU
    concurrency control" (Documentation/locking/locktypes.rst)
    
    local_lock protects against migration between when the percpu pointer is
    accessed and the pcp->lock acquired.  The lock acquisition is a preemption
    point so in the worst case, a task could migrate to another NUMA node and
    accidentally allocate remote memory.  The main requirement is to pin the
    task to a CPU that is suitable for PREEMPT_RT and !PREEMPT_RT.