Messages in this thread | | | Date | Tue, 28 Mar 2023 16:04:29 +0200 | From | Petr Tesařík <> | Subject | Re: [PATCH v2 1/1] swiotlb: Track and report io_tlb_used high water mark in debugfs |
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On Tue, 28 Mar 2023 15:50:17 +0200 Petr Tesařík <petr@tesarici.cz> wrote:
> On Tue, 28 Mar 2023 13:12:13 +0000 > "Michael Kelley (LINUX)" <mikelley@microsoft.com> wrote: > > > From: Christoph Hellwig <hch@infradead.org> 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. > > For my purposes, it does not have to be 100% accurate.
Actually, why are these variables global? There can be multiple io_tlb_mem instances in the system (one SWIOTLB and multiple restricted DMA pools). Tracking the usage of restricted DMA pools might be useful, but summing them up with the SWIOTLB not so much. AFAICS the watermark should be added to struct io_tlb_mem.
Petr T
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