linux-kernel - Re: [PATCH] mmap_lock: add tracepoints around lock acquisition

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Date:   Tue, 22 Sep 2020 12:51:13 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Yafang Shao <laoar.shao@...il.com>
Cc:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michel Lespinasse <walken@...gle.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Davidlohr Bueso <dbueso@...e.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH] mmap_lock: add tracepoints around lock acquisition

On Tue, 22 Sep 2020 12:09:19 +0800
Yafang Shao <laoar.shao@...il.com> wrote:

> > > Are there any methods to avoid un-inlining these wrappers ?
> > >
> > > For example,
> > > // include/linux/mmap_lock.h
> > >
> > > void mmap_lock_start_trace_wrapper();
> > > void mmap_lock_acquire_trace_wrapper();
> > >
> > > static inline void mmap_write_lock(struct mm_struct *mm)
> > > {
> > >     mmap_lock_start_trace_wrapper();
> > >     down_write(&mm->mmap_lock);
> > >     mmap_lock_acquire_trace_wrapper();
> > > }
> > >
> > > // mm/mmap_lock.c
> > > void mmap_lock_start_trace_wrapper()
> > > {
> > >     trace_mmap_lock_start();
> > > }
> > >
> > > void mmap_lock_start_trace_wrapper()
> > > {
> > >     trace_mmap_lock_acquired();
> > > }  
> >
> > We can do something like that, but I don't think it would end up being better.
> >
> > At the end of the day, because the trace stuff cannot be in the
> > header, we have to add an extra function call one way or the other.
> > This would just move the call one step further down the call stack.
> > So, I don't think it would affect performance in the
> > CONFIG_MMAP_LOCK_STATS + tracepoints not enabled at runtime case.
> >  
> 
> Right, it seems we have to add an extra function call.
> 
> > Also the wrappers aren't quite so simple as this, they need some
> > parameters to work. (the struct mm_struct, whether it was a read or a
> > write lock, and whether or not the lock operation succeeded), so it
> > would mean adding more inlined code, which I think adds up to be a
> > nontrivial amount since these wrappers are called so often in the
> > kernel.
> >
> > If you feel strongly, let me know and I can send a version as you
> > describe and we can compare the two.
> >  
> 
> These tracepoints will be less useful if we have to turn on the config
> to enable it.
> I don't mind implementing it that way if we can't optimize it.
> 
> Maybe Steven can give some suggestions, Steven ?
> 


What you can do, and what we have done is the following:

(see include/linux/page_ref.h)


#ifdef CONFIG_TRACING
extern struct tracepoint __tracepoint_mmap_lock_start_locking;
extern struct tracepoint __tracepoint_mmap_lock_acquire_returned;

#define mmap_lock_tracepoint_active(t) static_key_false(&(__tracepoint_mmap_lock_##t).key)

#else
#define mmap_lock_tracepoint_active(t) false
#endif

static inline void mmap_write_lock(struct mm_struct *mm)
{
	if (mmap_lock_tracepoint_active(start_locking))
		mmap_lock_start_trace_wrapper();
	down_write(&mm->mmap_lock);
	if (mmap_lock_tracepoint_active(acquire_returned))
		mmap_lock_acquire_trace_wrapper();
}


-- Steve