[PATCH v2 0/2] execve scalability issues, part 1

From: Mateusz Guzik
Date: Tue Aug 22 2023 - 14:44:11 EST


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 (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?

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 | 20 ++++++++---
kernel/fork.c | 14 ++------
lib/percpu_counter.c | 61 +++++++++++++++++++++++-----------
3 files changed, 60 insertions(+), 35 deletions(-)

--
2.39.2