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

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Date:   Mon, 26 Jun 2017 11:55:26 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     akpm@...ux-foundation.org, jeyu@...hat.com, shuah@...nel.org,
        rusty@...tcorp.com.au, ebiederm@...ssion.com,
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        josh@...htriplett.org, martin.wilck@...e.com, mmarek@...e.com,
        hare@...e.com, rwright@....com, jeffm@...e.com, DSterba@...e.com,
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        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 2017-06-23 18:16:19, 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
> > > @@ -163,14 +163,11 @@ int __request_module(bool wait, const char *fmt, ...)
> > >  		return ret;
> > >  
> > >  	if (atomic_dec_if_positive(&kmod_concurrent_max) < 0) {
> > > -		/* We may be blaming an innocent here, but unlikely */
> > > -		if (kmod_loop_msg < 5) {
> > > -			printk(KERN_ERR
> > > -			       "request_module: runaway loop modprobe %s\n",
> > > -			       module_name);
> > > -			kmod_loop_msg++;
> > > -		}
> > > -		return -ENOMEM;
> > > +		pr_warn_ratelimited("request_module: kmod_concurrent_max (%u) close to 0 (max_modprobes: %u), for module %s\n, throttling...",
> > > +				    atomic_read(&kmod_concurrent_max),
> > > +				    50, module_name);
> > 
> > It is weird to pass the constant '50' via %s.
> 
> The 50 was passed with %u, so I take it you meant it is odd to use a parameter
> for it.

Yeah, I meant %u and not %s.

> > Also a #define should be
> > used to keep it in sync with the kmod_concurrent_max initialization.
> 
> OK.
> 
> > > +		wait_event_interruptible(kmod_wq,
> > > +					 atomic_dec_if_positive(&kmod_concurrent_max) >= 0);
> > >  	}
> > >  
> > >  	trace_module_request(module_name, wait, _RET_IP_);
> > > @@ -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 think that we must wake_up() always when we increment
kmod_concurrent_max. If the value was negative, the increment will
allow exactly one process to pass that
atomic_dec_if_positive(&kmod_concurrent_max) >= 0). It the value
is positive, there must have been other wake_up() calls or there
is no waiter.

IMHO, this works because kmod_concurrent_max handling is atomic
and race-less now. Also (s)wait_event_interruptible() is safe
and does not allow to get into sleep when the resource is available.

Anyway, it is great that you have double checked this.

Best Regards,
Petr