On Mon, 25 Nov 2024 at 15:44, Chris Bainbridge
<chris.bainbridge@xxxxxxxxx> wrote:
>
> The commit 6975c1a486a4 ("block: remove the ioprio field from struct
> request") appears to have introduced a performance regression. Test
> case is the script /etc/cron.daily/locate from package `locate` (which
> is from findutils locate on Debian). The script runs:
I did a bit of debugging.
The problem is the function req_get_ioprio. The commit changed:
static inline unsigned short req_get_ioprio(struct request *req)
{
- return req->ioprio;
+ if (req->bio)
+ return req->bio->bi_ioprio;
+ return 0;
}
So when req->bio is NULL this function now returns 0. But previously the values
of req->ioprio were sometimes non-zero. If I revert the commit and then
instrument req_get_ioprio to log where the new return value differs:
static inline unsigned short req_get_ioprio(struct request *req)
{
- return req->ioprio;
+ if (req->bio)
+ {
+ if (req->bio->bi_ioprio != req->ioprio)
+ printk("req->bio->bi_ioprio != req->ioprio\n");
+ return req->bio->bi_ioprio;
+ }
+ if (req->ioprio != 0)
+ printk("bad ioprio 0 != %u\n", (unsigned int)req->ioprio);
+ return 0;
}
then log shows:
[ 36.922906] bad ioprio 0 != 16387
[ 36.923061] bad ioprio 0 != 16387
[ 36.930186] bad ioprio 0 != 16387
[ 36.930680] bad ioprio 0 != 16387
[ 78.875421] bad ioprio 0 != 24583
[ 79.228801] bad ioprio 0 != 24583
[ 87.411118] bad ioprio 0 != 24583
[ 97.419607] bad ioprio 0 != 24583
[ 97.421059] bad ioprio 0 != 24583
[ 107.210364] bad ioprio 0 != 24583
[ 107.210775] bad ioprio 0 != 24583
So, the new function is returning 0 when it would've previously returned these
non-zero values, and returning zero breaks things.
