From: Mikulas Patocka <mpatocka@redhat.com>
To: Hannes Reinecke <hare@suse.de>
Cc: Damien Le Moal <dlemoal@kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Tejun Heo <tj@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
linux-kernel@vger.kernel.org, dm-devel@lists.linux.dev,
Mike Snitzer <msnitzer@redhat.com>,
Ignat Korchagin <ignat@cloudflare.com>,
Damien Le Moal <damien.lemoal@wdc.com>,
Hou Tao <houtao1@huawei.com>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>
Subject: Re: [PATCH] softirq: fix memory corruption when freeing tasklet_struct
Date: Fri, 26 Jan 2024 19:09:31 +0100 (CET) [thread overview]
Message-ID: <d01a8554-10dd-1968-c9b0-f9ff16e6ac4@redhat.com> (raw)
In-Reply-To: <d390d7ee-f142-44d3-822a-87949e14608b@suse.de>
On Fri, 26 Jan 2024, Hannes Reinecke wrote:
> Oh, it's this time of the year again?
> (This topic regularly comes up ...)
>
> The reason is not that it will disable dm-multipath (Mike Snitzer put in
> bio-based multipathing as an additional code path); the reason is that
> dm-multipath performance will suffer when you remove request-based DM.
Is there some benchmark that says how much will it suffer?
> DM-multipath schedules based on request (if you use the request-based
> interface) or bios (if you use the bio-based interface).
> Any merge decision is typically done by the block layer when combining bios
> into requests; and you can only merge bios if the bvecs are adjacent.
> So if you use bio-based multipathing you will spread sequential bios
> across all paths, leaving the block layer unable to merge requests.
The same problem exists in raid1 and there's a function read_balance that
solves it. If the starting sector of a new bio matches the ending sector
of a previous bio, then submit it for the same device.
> For request based multipathing the requests are already fully-formed,
> and scheduling across paths does not change them.
> Things are slightly better with multi-page bvecs nowadays, but the
> overall picture still stands.
>
> Another thing is timeouts; bios don't do timeouts, so a bio can run
> for an arbitrary time with no chance of interrupting it.
> Requests do have a timeout, and will be aborted from the driver when
> the timeout is hit.
> Seeing that 99% of all I/O issues I've seen _are_ timeouts it becomes
> a crucial feature if you want dm-multipath to control failover time.
You can set timeout of the underlying physical devices.
> Cheers,
>
> Hannes
Mikulas
next prev parent reply other threads:[~2024-01-26 18:09 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-01-25 18:29 [PATCH] softirq: fix memory corruption when freeing tasklet_struct Mikulas Patocka
2024-01-25 19:51 ` Linus Torvalds
2024-01-25 22:04 ` Mikulas Patocka
2024-01-25 23:15 ` Damien Le Moal
2024-01-26 6:56 ` Hannes Reinecke
2024-01-26 18:09 ` Mikulas Patocka [this message]
2024-01-29 8:26 ` request based dm-multipath, was " Christoph Hellwig
2024-01-25 22:51 ` Ignat Korchagin
2024-01-26 0:33 ` Tejun Heo
2024-01-26 18:36 ` Allen
2024-01-26 23:43 ` Tejun Heo
2024-01-27 3:13 ` Allen
2024-01-27 18:37 ` Tejun Heo
2024-01-30 9:19 ` Tejun Heo
2024-01-29 17:00 ` Allen
2024-01-29 17:06 ` Tejun Heo
2024-04-09 14:15 ` Li Lingfeng
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