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

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Date:   Mon, 21 Aug 2023 22:28:27 +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, linux-mm@...ck.org,
        Mateusz Guzik <mjguzik@...il.com>
Subject: [PATCH 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 found 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 (warning: first line is garbage).

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.

Thoughts?

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

 include/linux/percpu_counter.h | 19 ++++++++---
 kernel/fork.c                  | 13 ++------
 lib/percpu_counter.c           | 61 ++++++++++++++++++++++++----------
 3 files changed, 60 insertions(+), 33 deletions(-)

-- 
2.39.2