linux-kernel - Re: [RFC PATCH 0/1] Large folios in block buffered IO path

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Message-ID: <2220e327-5587-4d3c-917d-d0a2728a0f73@amd.com>
Date: Mon, 2 Dec 2024 15:07:23 +0530
From: Bharata B Rao <bharata@....com>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, nikunj@....com,
 willy@...radead.org, vbabka@...e.cz, david@...hat.com,
 akpm@...ux-foundation.org, yuzhao@...gle.com, axboe@...nel.dk,
 viro@...iv.linux.org.uk, brauner@...nel.org, jack@...e.cz,
 joshdon@...gle.com, clm@...a.com
Subject: Re: [RFC PATCH 0/1] Large folios in block buffered IO path

On 28-Nov-24 10:01 AM, Mateusz Guzik wrote:

> WIlly mentioned the folio wait queue hash table could be grown, you
> can find it in mm/filemap.c:
>    1062 #define PAGE_WAIT_TABLE_BITS 8
>    1063 #define PAGE_WAIT_TABLE_SIZE (1 << PAGE_WAIT_TABLE_BITS)
>    1064 static wait_queue_head_t folio_wait_table[PAGE_WAIT_TABLE_SIZE]
> __cacheline_aligned;
>    1065
>    1066 static wait_queue_head_t *folio_waitqueue(struct folio *folio)
>    1067 {
>    1068 │       return &folio_wait_table[hash_ptr(folio, PAGE_WAIT_TABLE_BITS)];
>    1069 }
> 
> Can you collect off cpu time? offcputime-bpfcc -K > /tmp/out

Flamegraph for "perf record --off-cpu -F 99 -a -g --all-kernel 
--kernel-callchains -- sleep 120" is attached.

Off-cpu samples were collected for 120s at around 45th minute run of the 
FIO benchmark that actually runs for 1hr. This run was with kernel that 
had your inode_lock fix but no changes to PAGE_WAIT_TABLE_BITS.

Hopefully this captures the representative sample of the scalability 
issue with folio lock.

Regards,
Bharata.
Download attachment "perf-inode-fix.svg" of type "image/svg+xml" (243647 bytes)