Re: [PATCH] mmap_lock: add tracepoints around lock acquisition

[Yafang Shao]

On Wed, Sep 23, 2020 at 12:51 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Tue, 22 Sep 2020 12:09:19 +0800
> Yafang Shao <laoar.shao@xxxxxxxxx> 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


Great!

Thanks Steve.

-- 
Thanks
Yafang