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

From: Luis R. Rodriguez
Date: Fri Jun 23 2017 - 13:56:27 EST

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.