Re: [PATCH] fs/select.c: batch user writes in do_sys_poll

From: Daniel Axtens
Date: Thu Aug 13 2020 - 04:48:30 EST

Christoph Hellwig <hch@xxxxxxxxxxxxx> writes:

> On Thu, Aug 13, 2020 at 05:11:20PM +1000, Daniel Axtens wrote:
>> When returning results to userspace, do_sys_poll repeatedly calls
>> put_user() - once per fd that it's watching.
>> This means that on architectures that support some form of
>> kernel-to-userspace access protection, we end up enabling and disabling
>> access once for each file descripter we're watching. This is inefficent
>> and we can improve things by batching the accesses together.
>> To make sure there's not too much happening in the window when user
>> accesses are permitted, we don't walk the linked list with accesses on.
>> This leads to some slightly messy code in the loop, unfortunately.
>> Unscientific benchmarking with the poll2_threads microbenchmark from
>> will-it-scale, run as `./poll2_threads -t 1 -s 15`:
>> - Bare-metal Power9 with KUAP: ~48.8% speed-up
>> - VM on amd64 laptop with SMAP: ~25.5% speed-up
>> Signed-off-by: Daniel Axtens <dja@xxxxxxxxxx>
> Seem like this could simply use a copy_to_user to further simplify
> things?

I'll benchmark it and find out.

> Also please don't pointlessly add overly long lines.

Weird, I ran the commit through checkpatch and it didn't pick it
up. I'll check the next version more carefully.