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

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Date: Wed, 1 May 2024 20:31:06 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Masami Hiramatsu <mhiramat@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	kuba@...nel.org, ast@...nel.org, clm@...com, rostedt@...dmis.org,
	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 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.

							Thanx, Paul

> 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>