linux-kernel - Re: [PATCH resend ftrace] Asynchronous grace period for register_ftrace

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Message-ID: <135a21d0-1622-418a-b49f-8338a4230ff4@paulmck-laptop>
Date: Thu, 2 May 2024 16:13:59 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org, kuba@...nel.org, ast@...nel.org,
	clm@...com, mark.rutland@....com, mathieu.desnoyers@...icios.com
Subject: Re: [PATCH resend ftrace] Asynchronous grace period for
 register_ftrace_direct()

On Thu, May 02, 2024 at 05:31:00PM -0400, Steven Rostedt wrote:
> On Wed, 1 May 2024 20:31:06 -0700
> "Paul E. McKenney" <paulmck@...nel.org> wrote:
> 
> > On Thu, May 02, 2024 at 11:05:01AM +0900, Masami Hiramatsu wrote:
> > > On Wed, 1 May 2024 16:12:37 -0700
> > > "Paul E. McKenney" <paulmck@...nel.org> wrote:
> > >   
> > > > Note that the immediate pressure for this patch should be relieved by the
> > > > NAPI patch series [1], but this sort of problem could easily arise again.
> > > > 
> > > > When running heavy test workloads with KASAN enabled, RCU Tasks grace
> > > > periods can extend for many tens of seconds, significantly slowing
> > > > trace registration.  Therefore, make the registration-side RCU Tasks
> > > > grace period be asynchronous via call_rcu_tasks().  
> > > 
> > > Good catch! AFAICS, there is no reason to wait for synchronization
> > > when adding a new direct trampoline.
> > > This looks good to me.
> > > 
> > > Reviewed-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>  
> > 
> > Thank you very much!  I will apply this on my next rebase.
> 
> I can take it.
> 
> It's not a bug fix but just an performance improvement, so it can go into
> the next merge window.

Very good, and thank you!

I will drop it from RCU as soon as it shows up in either -next or in
mainline.

							Thanx, Paul

> -- Steve
> 
> 
> 
> > 
> > > Thank you,
> > >   
> > > > [1]
> > > > https://lore.kernel.org/all/cover.1710877680.git.yan@cloudflare.com/
> > > > 
> > > > Reported-by: Jakub Kicinski <kuba@...nel.org>
> > > > Reported-by: Alexei Starovoitov <ast@...nel.org>
> > > > Reported-by: Chris Mason <clm@...com>
> > > > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > > > Cc: Steven Rostedt <rostedt@...dmis.org>
> > > > Cc: Masami Hiramatsu <mhiramat@...nel.org>
> > > > Cc: Mark Rutland <mark.rutland@....com>
> > > > Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> > > > Cc: <linux-trace-kernel@...r.kernel.org>
> > > > 
> > > > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> > > > index 6c96b30f3d63b..32ea92934268c 100644
> > > > --- a/kernel/trace/ftrace.c
> > > > +++ b/kernel/trace/ftrace.c
> > > > @@ -5365,6 +5365,13 @@ static void
> > > > remove_direct_functions_hash(struct ftrace_hash *hash, unsigned long }
> > > >  }
> > > >  
> > > > +static void register_ftrace_direct_cb(struct rcu_head *rhp)
> > > > +{
> > > > +	struct ftrace_hash *fhp = container_of(rhp, struct
> > > > ftrace_hash, rcu); +
> > > > +	free_ftrace_hash(fhp);
> > > > +}
> > > > +
> > > >  /**
> > > >   * register_ftrace_direct - Call a custom trampoline directly
> > > >   * for multiple functions registered in @ops
> > > > @@ -5463,10 +5470,8 @@ int register_ftrace_direct(struct ftrace_ops
> > > > *ops, unsigned long addr) out_unlock:
> > > >  	mutex_unlock(&direct_mutex);
> > > >  
> > > > -	if (free_hash && free_hash != EMPTY_HASH) {
> > > > -		synchronize_rcu_tasks();
> > > > -		free_ftrace_hash(free_hash);
> > > > -	}
> > > > +	if (free_hash && free_hash != EMPTY_HASH)
> > > > +		call_rcu_tasks(&free_hash->rcu,
> > > > register_ftrace_direct_cb); 
> > > >  	if (new_hash)
> > > >  		free_ftrace_hash(new_hash);  
> > > 
> > > 
> > > -- 
> > > Masami Hiramatsu (Google) <mhiramat@...nel.org>  
>