RE: [PATCH v2 1/1] swiotlb: Track and report io_tlb_used high water mark in debugfs

[Michael Kelley (LINUX)]

From: Christoph Hellwig <hch@xxxxxxxxxxxxx> Sent: Monday, March 27, 2023 6:34 PM
> 
> On Sat, Mar 25, 2023 at 10:53:10AM -0700, Michael Kelley wrote:
> > @@ -659,6 +663,14 @@ static int swiotlb_do_find_slots(struct device *dev, int
> area_index,
> >  	area->index = wrap_area_index(mem, index + nslots);
> >  	area->used += nslots;
> >  	spin_unlock_irqrestore(&area->lock, flags);
> > +
> > +	new_used = atomic_long_add_return(nslots, &total_used);
> > +	old_hiwater = atomic_long_read(&used_hiwater);
> > +	do {
> > +		if (new_used <= old_hiwater)
> > +			break;
> > +	} while (!atomic_long_try_cmpxchg(&used_hiwater, &old_hiwater, new_used));
> > +
> >  	return slot_index;
> 
> Hmm, so we're right in the swiotlb hot path here and add two new global
> atomics?

It's only one global atomic, except when the high water mark needs to be
bumped.  That results in an initial transient of doing the second global
atomic, but then it won't be done unless there's a spike in usage or the
high water mark is manually reset to zero.  Of course, there's a similar
global atomic subtract when the slots are released.

Perhaps this accounting should go under #ifdef CONFIG_DEBUGFS?  Or
even add a swiotlb-specific debugfs config option to cover all the swiotlb
debugfs code.  From Petr Tesarik's earlier comments, it sounds like there
is interest in additional accounting, such as for fragmentation.

> 
> >  static int io_tlb_used_get(void *data, u64 *val)
> >  {
> > -	*val = mem_used(&io_tlb_default_mem);
> > +	*val = (u64)atomic_long_read(&total_used);
> >  	return 0;
> >  }
> > +
> > +static int io_tlb_hiwater_get(void *data, u64 *val)
> > +{
> > +	*val = (u64)atomic_long_read(&used_hiwater);
> 
> I can't see how these casts would be needed.

OK.  Will drop the casts in the next version.

Michael