From: Saeed Mahameed <saeedm@mellanox.com>
To: "jonathan.lemon@gmail.com" <jonathan.lemon@gmail.com>,
"eric.dumazet@gmail.com" <eric.dumazet@gmail.com>
Cc: "netdev@vger.kernel.org" <netdev@vger.kernel.org>
Subject: Re: rtnl_lock() question
Date: Wed, 4 Sep 2019 23:23:43 +0000 [thread overview]
Message-ID: <867cf373f204715aec3b2e04ef9f65454cf25a2e.camel@mellanox.com> (raw)
In-Reply-To: <C46053D2-6BF5-4CFE-BF76-32DDCAD7BC10@gmail.com>
On Wed, 2019-09-04 at 09:38 -0700, Jonathan Lemon wrote:
> On 4 Sep 2019, at 0:39, Eric Dumazet wrote:
>
> > On 9/3/19 11:55 PM, Jonathan Lemon wrote:
> > > How appropriate is it to hold the rtnl_lock() across a sleepable
> > > memory allocation? On one hand it's just a mutex, but it would
> > > seem like it could block quite a few things.
> > >
> >
> > Sure, all GFP_KERNEL allocations can sleep for quite a while.
> >
> > On the other hand, we may want to delay stuff if memory is under
> > pressure,
> > or complex operations like NEWLINK would fail.
> >
> > RTNL is mostly taken for control path operations, we prefer them to
> > be
> > mostly reliable, otherwise admins job would be a nightmare.
> >
> > In some cases, it is relatively easy to pre-allocate memory before
> > rtnl is taken,
> > but that will only take care of some selected paths.
>
> The particular code path that I'm looking at is
> mlx5e_tx_timeout_work().
>
> This is called on TX timeout, and mlx5 wants to move an entire
> channel
> and all the supporting structures elsewhere. Under the rtnl_lock(),
> it
> calls kvzmalloc() in order to grab a large chunk of contig memory,
> which
> ends up stalling the system.
>
> I suspect these large allocation should really be done outside the
> lock.
I am afraid that is impossible, at least not for all allocations
some allocations require parameters that should remain valid and
constant across the whole reconfiguration procedure such
params.num_channels, so they must be done inside the lock.
other allocations are buried deep inside mlx5 that by doing pre
allocations is going to require a lot of refactoring.
One idea is to use some sort of mem cache specifically for mlx5
reconfiguration that is cheaper to call than raw kvzalloc ? but
different objects in the mlx5 reconfiguration path requires differnt
memory types, numa affinity etc.. which might make the cache harder to
satisfy all requirements.
next prev parent reply other threads:[~2019-09-04 23:23 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-03 21:55 rtnl_lock() question Jonathan Lemon
2019-09-04 7:39 ` Eric Dumazet
2019-09-04 16:38 ` Jonathan Lemon
2019-09-04 23:23 ` Saeed Mahameed [this message]
2019-09-05 18:07 ` Rustad, Mark D
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