linux-kernel - Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC

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Date:   Thu, 21 Apr 2022 15:44:08 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of
 ARCH_KMALLOC_MINALIGN

On Thu, Apr 21, 2022 at 03:47:30PM +0200, Arnd Bergmann wrote:
> On Thu, Apr 21, 2022 at 3:25 PM Catalin Marinas <catalin.marinas@....com> wrote:
> > On Thu, Apr 21, 2022 at 02:28:45PM +0200, Arnd Bergmann wrote:
> > > We also know that larger slabs are all cacheline aligned, so simply
> > > comparing the transfer size is enough to rule out most, in this case
> > > any transfer larger than 96 bytes must come from the kmalloc-128
> > > or larger cache, so that works like before.
> >
> > There's also the case with 128-byte cache lines and kmalloc-192.
> 
> Sure, but that's much less common, as the few machines with 128 byte
> cache lines tend to also have cache coherent devices IIRC, so we'd
> skip the bounce buffer entirely.

Do you know which machines still have 128-byte cache lines _and_
non-coherent DMA? If there isn't any that matters, I'd reduce
ARCH_DMA_MINALIGN to 64 now (while trying to get to even smaller kmalloc
caches).

> > > For transfers <=96 bytes, the possibilities are:
> > >
> > > 1.kmalloc-32 or smaller, always needs to bounce
> > > 2. kmalloc-96, but at least one byte in partial cache line,
> > >     need to bounce
> > > 3. kmalloc-64, may skip the bounce.
> > > 4. kmalloc-128 or larger, or not a slab cache but a partial
> > >     transfer, may skip the bounce.
> > >
> > > I would guess that the first case is the most common here,
> > > so unless bouncing one or two cache lines is extremely
> > > expensive, I don't expect it to be worth optimizing for the latter
> > > two cases.
> >
> > I think so. If someone complains of a performance regression, we can
> > look at optimising the bounce. I have a suspicion the cost of copying
> > two cache lines is small compared to swiotlb_find_slots() etc.
> 
> That is possible, and we'd definitely have to watch out for
> performance regressions, I'm just skeptical that the cases that
> suffer from the extra bouncer buffering on 33..64 byte allocations
> benefit much from having a special case if the 1...32 and 65..96
> byte allocations are still slow.
> 
> Another simpler way to do this might be to just not create the
> kmalloc-96 (or kmalloc-192) caches, and assuming that any
> transfer >=33 (or 65) bytes is safe.

I'll give the dma bounce idea a go next week, see how it looks.

-- 
Catalin