linux-kernel - [PATCH v3 0/2] execve scalability issues, part 1

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Date:   Wed, 23 Aug 2023 07:06:07 +0200
From:   Mateusz Guzik <mjguzik@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     dennis@...nel.org, tj@...nel.org, cl@...ux.com,
        akpm@...ux-foundation.org, shakeelb@...gle.com,
        vegard.nossum@...cle.com, linux-mm@...ck.org,
        Mateusz Guzik <mjguzik@...il.com>
Subject: [PATCH v3 0/2] execve scalability issues, part 1

To start I figured I'm going to bench about as friendly case as it gets
-- statically linked *separate* binaries all doing execve in a loop.

I borrowed the bench from here:
http://apollo.backplane.com/DFlyMisc/doexec.c

$ cc -static -O2 -o static-doexec doexec.c
$ ./static-doexec $(nproc)

It prints a result every second.

My test box is temporarily only 26 cores and even at this scale I run
into massive lock contention stemming from back-to-back calls to
percpu_counter_init (and _destroy later).

While not a panacea, one simple thing to do here is to batch these ops.
Since the term "batching" is already used in the file, I decided to
refer to it as "grouping" instead.

Even if this code could be patched to dodge these counters,  I would
argue a high-traffic alloc/free consumer is only a matter of time so it
makes sense to facilitate it.

With the fix I get an ok win, to quote from the commit:
> Even at a very modest scale of 26 cores (ops/s):
> before: 133543.63
> after:  186061.81 (+39%)

While with the patch these allocations remain a significant problem,
the primary bottleneck shifts to:

    __pv_queued_spin_lock_slowpath+1
    _raw_spin_lock_irqsave+57
    folio_lruvec_lock_irqsave+91
    release_pages+590
    tlb_batch_pages_flush+61
    tlb_finish_mmu+101
    exit_mmap+327
    __mmput+61
    begin_new_exec+1245
    load_elf_binary+712
    bprm_execve+644
    do_execveat_common.isra.0+429
    __x64_sys_execve+50
    do_syscall_64+46
    entry_SYSCALL_64_after_hwframe+110

I intend to do more work on the area to mostly sort it out, but I would
not mind if someone else took the hammer to folio. :)

With this out of the way I'll be looking at some form of caching to
eliminate these allocs as a problem.

v3:
- fix !CONFIG_SMP build
- drop the backtrace from fork commit message

v2:
- force bigger alignment on alloc
- rename "counters" to "nr_counters" and pass prior to lock key
- drop {}'s for single-statement loops

Mateusz Guzik (2):
  pcpcntr: add group allocation/free
  fork: group allocation of per-cpu counters for mm struct

 include/linux/percpu_counter.h | 39 ++++++++++++++++++----
 kernel/fork.c                  | 14 ++------
 lib/percpu_counter.c           | 61 +++++++++++++++++++++++-----------
 3 files changed, 77 insertions(+), 37 deletions(-)

-- 
2.41.0