linux-kernel - Re: [PATCH v3 4/4] kmod: throttle kmod thread limit

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Message-ID: <20170623175611.GP21846@wotan.suse.de>
Date:   Fri, 23 Jun 2017 19:56:11 +0200
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     Petr Mladek <pmladek@...e.com>, akpm@...ux-foundation.org,
        jeyu@...hat.com, shuah@...nel.org, rusty@...tcorp.com.au,
        ebiederm@...ssion.com, dmitry.torokhov@...il.com, acme@...hat.com,
        corbet@....net, josh@...htriplett.org, martin.wilck@...e.com,
        mmarek@...e.com, hare@...e.com, rwright@....com, jeffm@...e.com,
        DSterba@...e.com, fdmanana@...e.com, neilb@...e.com,
        linux@...ck-us.net, rgoldwyn@...e.com, subashab@...eaurora.org,
        xypron.glpk@....de, keescook@...omium.org, atomlin@...hat.com,
        mbenes@...e.cz, paulmck@...ux.vnet.ibm.com,
        dan.j.williams@...el.com, jpoimboe@...hat.com, davem@...emloft.net,
        mingo@...hat.com, alan@...ux.intel.com, tytso@....edu,
        gregkh@...uxfoundation.org, torvalds@...ux-foundation.org,
        linux-kselftest@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 4/4] kmod: throttle kmod thread limit

On Fri, Jun 23, 2017 at 06:16:19PM +0200, Luis R. Rodriguez wrote:
> On Thu, Jun 22, 2017 at 05:19:36PM +0200, Petr Mladek wrote:
> > On Fri 2017-05-26 14:12:28, Luis R. Rodriguez wrote:
> > > --- a/kernel/kmod.c
> > > +++ b/kernel/kmod.c
> > > @@ -178,6 +175,7 @@ int __request_module(bool wait, const char *fmt, ...)
> > >  	ret = call_modprobe(module_name, wait ? UMH_WAIT_PROC : UMH_WAIT_EXEC);
> > >  
> > >  	atomic_inc(&kmod_concurrent_max);
> > > +	wake_up_all(&kmod_wq);
> > 
> > Does it make sense to wake up all waiters when we released the resource
> > only for one? IMHO, a simple wake_up() should be here.
> 
> Then we should wake_up() also on failure, otherwise we have the potential
> to not wake some in a proper time.

I checked and it turns out we have no error paths after we consume a kmod
ticket, if you will. Once we bump with atomic_dec_if_positive() we assume
we're moving forward with an attempt, and the only failure path is already
bundled with a wake at the end of the __request_module() call.

Then the next question would be *who* exactly gets woken up next if we just
use wake_up() ? The common core wake up code varies depending on use and
all this reminded me of the complexity we just don't need, so I have now
converted to use swait. swait uses list_add() if empty and then iterates
with list_first_entry() on wakeup, so that should get the first item added
to the wait list.

Works with me. Will run a test a before v4 is sent, but since only 2 patches
are modified will only send a respective update for these 2 patches.

  Luis