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Date: Tue, 27 Aug 2024 17:30:59 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Petr Tesařík <petr@...arici.cz>
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Subject: RE: [RFC 1/7] swiotlb: Introduce swiotlb throttling

From: Petr Tesařík <petr@...arici.cz> Sent: Tuesday, August 27, 2024 8:56 AM
> 
> On Fri, 23 Aug 2024 20:41:15 +0000
> Michael Kelley <mhklinux@...look.com> wrote:
> 
> > From: Petr Tesařík <petr@...arici.cz> Sent: Friday, August 23, 2024 12:41 AM
> > >
> > > On Thu, 22 Aug 2024 11:37:12 -0700
> > > mhkelley58@...il.com wrote:
> >[...]
> > > > @@ -71,12 +72,15 @@
> > > >   *		from each index.
> > > >   * @pad_slots:	Number of preceding padding slots. Valid only in the first
> > > >   *		allocated non-padding slot.
> > > > + * @throttled:  Boolean indicating the slot is used by a request that was
> > > > + *		throttled. Valid only in the first allocated non-padding slot.
> > > >   */
> > > >  struct io_tlb_slot {
> > > >  	phys_addr_t orig_addr;
> > > >  	size_t alloc_size;
> > > >  	unsigned short list;
> > > > -	unsigned short pad_slots;
> > > > +	u8 pad_slots;
> > > > +	u8 throttled;
> > >
> > > I'm not sure this flag is needed for each slot.
> > >
> > > SWIOTLB mappings should be throttled when the total SWIOTLB usage is
> > > above a threshold. Conversely, it can be unthrottled when the total
> > > usage goes below a threshold, and it should not matter if that happens
> > > due to an unmap of the exact buffer which previously pushed the usage
> > > over the edge, or due to an unmap of any other unrelated buffer.
> >
> > I think I understand what you are proposing. But I don't see a way
> > to make it work without adding global synchronization beyond
> > the current atomic counter for the number of uI'm sed slabs. At a minimum
> > we would need a global spin lock instead of the atomic counter. The spin
> > lock would protect the (non-atomic) slab count along with some other
> > accounting, and that's more global references. As described in the
> > cover letter, I was trying to avoid doing that.
> 
> I have thought about this for a few days. And I'm still not convinced.
> You have made it clear in multiple places that the threshold is a soft
> limit, and there are many ways the SWIOTLB utilization may exceed the
> threshold. In fact I'm not even 100% sure that an atomic counter is
> needed, because the check is racy anyway.

Atomic operations are expensive at the memory bus level, particularly
in high CPU count systems with NUMA topologies. However,
maintaining an imprecise global count doesn't work because the
divergence from reality can become unbounded over time. The
alternative is to sum up all the per-area counters each time a
reasonably good global value is needed, and that can be expensive itself
with high area counts. A hybrid might be to maintain an imprecise global
count, but periodically update it by summing up all the per-area counters
so that the divergence from reality isn't unbounded.

> Another task may increase
> (or decrease) the counter between atomic_long_read(&mem->total_used)
> and a subsequent down(&mem->throttle_sem).
> 
> I consider it a feature, not a flaw, because the real important checks
> happen later while searching for free slots, and those are protected
> with a spinlock.
> 
> > If you can see how to do what you propose with just the current
> > atomic counter, please describe.
> 
> I think I'm certainly missing something obvious, but let me open the
> discussion to improve my understanding of the matter.
> 
> Suppose we don't protect the slab count with anything. What is the
> worst possible outcome? IIUC the worst scenario is that multiple tasks
> unmap swiotlb buffers simultaneously and all of them believe that their
> action made the total usage go below the low threshold, so all of them
> try to release the semaphore.
> 
> That's obviously not good, but AFAICS all that's needed is a
> test_and_clear_bit() on a per-io_tlb_mem throttled flag just before
> calling up(). Since up() would acquire the semaphore's spinlock, and
> there's only one semaphore per io_tlb_mem, adding an atomic flag doesn't
> look like too much overhead to me, especially if it ends up in the same
> cache line as the semaphore.

Yes, the semaphore management is the problem. Presumably we want
each throttled request to wait on the semaphore, forming an ordered
queue of waiters. Each up() on the semaphore releases one of those
waiters. We don’t want to release all the waiters when the slab count
transitions from "above throttle" to "below throttle" because that
creates a thundering herd problem.

So consider this example scenario:
1) Two waiters ("A" and "B") are queued the semaphore, each wanting 2 slabs.
2) An unrelated swiotlb unmap frees 10 slabs, taking the slab count
from 2 above threshold to 8 below threshold. This does up() on
the semaphore and awakens "A".
3) "A" does his request for 2 slabs, and the slab count is now 6 below
threshold.
4) "A" does swiotlb unmap.  The slab count goes from 6 below threshold back
to 8 below threshold, so no semaphore operation is done. "B" is still waiting.
5) System-wide, swiotlb requests decline, and the slab count never goes above
the threshold again. At this point, "B" is still waiting and never gets awakened.

An ordered queue of waiters is incompatible with wakeups determined solely
on whether the slab count is below the threshold after swiotlb unmap. You
would have to wait up all waiters and let them re-contend for the slots that
are available below the threshold, with most probably losing out and going
back on the semaphore wait queue (i.e., a thundering herd).

Separately, what does a swiotlb unmap do if it takes the slab count from above
threshold to below threshold, and there are no waiters? It should not do up()
in that case, but how can it make that decision in a way that doesn't race
with a swiotlb map operation running at the same time?

Michael

> 
> Besides, this all happens only in the slow path, i.e. only after the
> current utilization has just dropped below the low threshold, not if
> the utilization was already below the threshold before freeing up some
> slots.
> 
> I have briefly considered subtracting the low threshold as initial bias
> from mem->total_used and using atomic_long_add_negative() to avoid the
> need for an extra throttled flag, but at this point I'm not sure it can
> be implemented without any races. We can try to figure out the details
> if it sounds like a good idea.
> 
> Petr T

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