Re: [PATCH] Revert "arm64: Increase the max granular size"

[Catalin Marinas]

On Tue, Apr 18, 2017 at 10:35:02PM +0530, Sunil Kovvuri wrote:
> On Tue, Apr 18, 2017 at 8:18 PM, Catalin Marinas
> <catalin.marinas@xxxxxxx> wrote:
> > On Mon, Apr 17, 2017 at 04:08:52PM +0530, Sunil Kovvuri wrote:
> >> >>     >> Do you have an explanation on the performance variation when
> >> >>     >> L1_CACHE_BYTES is changed? We'd need to understand how the network stack
> >> >>     >> is affected by L1_CACHE_BYTES, in which context it uses it (is it for
> >> >>     >> non-coherent DMA?).
> >> >>     >
> >> >>     > network stack use SKB_DATA_ALIGN to align.
> >> >>     > ---
> >> >>     > #define SKB_DATA_ALIGN(X) (((X) + (SMP_CACHE_BYTES - 1)) & \
> >> >>     > ~(SMP_CACHE_BYTES - 1))
> >> >>     >
> >> >>     > #define SMP_CACHE_BYTES L1_CACHE_BYTES
> >> >>     > ---
> >> >>     > I think this is the reason of performance regression.
> >> >>     >
> >> >>
> >> >>     Yes this is the reason for performance regression. Due to increases L1 cache alignment the
> >> >>     object is coming from next kmalloc slab and skb->truesize is changing from 2304 bytes to
> >> >>     4352 bytes. This in turn increases sk_wmem_alloc which causes queuing of less send buffers.
> >>
> >> With what traffic did you check 'skb->truesize' ? Increase from
> >> 2304 to 4352 bytes doesn't seem to be real. I checked with ICMP
> >> pkts with maximum size possible with 1500byte MTU and I don't see
> >> such a bump. If the bump is observed with Iperf sending TCP packets
> >> then I suggest to check if TSO is playing a part over here.
> >
> > I haven't checked truesize but I added some printks to __alloc_skb() (on
> > a Juno platform) and the size argument to this function is 1720 on many
> > occasions. With sizeof(struct skb_shared_info) of 320, the actual data
> > allocation is exactly 2048 when using 64 byte L1_CACHE_SIZE. With a
> > 128 byte cache size, it goes slightly over 2K, hence the 4K slab
> > allocation.
> 
> Understood but still in my opinion this '4K slab allocation' cannot be
> considered as an issue with cache line size, there are many network
> drivers out there which do receive buffer or page recycling to
> minimize (sometimes almost to zero) the cost of buffer allocation.

The slab allocation shouldn't make much difference (unless you are
running on a memory constrained system) but I don't understand how
skb->truesize (which is almost half unused) affects the sk_wmem_alloc
and its interaction with other bits in the network stack (e.g.
tcp_limit_output_bytes).

However, I do think it's worth investigating further to fully understand
the issue.

> >The 1720 figure surprised me a bit as well since I was
> > expecting something close to 1500.
> >
> > The thing that worries me is that skb->data may be used as a buffer to
> > DMA into. If that's the case, skb_shared_info is wrongly aligned based
> > on SMP_CACHE_BYTES only and can lead to corruption on a non-DMA-coherent
> > platform. It should really be ARCH_DMA_MINALIGN.
> 
> I didn't get this, if you see __alloc_skb()
> 
> 229         size = SKB_DATA_ALIGN(size);
> 230         size += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> 
> both DMA buffer and skb_shared_info are aligned to a cacheline separately,
> considering 128byte alignment guarantees 64byte alignment as well, how
> will this lead to corruption ?

It's the other way around: you align only to 64 byte while running on a
platform with 128 byte cache lines and non-coherent DMA.

> And if platform is non-DMA-coherent then again it's the driver which
> should take of coherency by using appropriate map/unmap APIs and
> should avoid any cacheline sharing btw DMA buffer and skb_shared_info.

The problem is that the streaming DMA API can only work correctly on
cacheline-aligned buffers (because of the cache invalidation it performs
for DMA ops; even with clean&invalidate, the operation isn't always safe
if a cacheline is shared between DMA and CPU buffers). In the skb case,
we could have the data potentially sharing the last addresses of a DMA
buffer with struct skb_shared_info.

We may be able to get away with SKB_DATA_ALIGN not using
ARCH_DMA_MINALIGN *if* skb_shared_info is *not* written before or during
an inbound DMA transfer (though such tricks are arch specific).

> > IIUC, the Cavium platform has coherent DMA, so it shouldn't be an issue
> > if we go back to 64 byte cache lines.
> 
> Yes, Cavium platform is DMA coherent and there is no issue with reverting back
> to 64byte cachelines. But do we want to do this because some platform has a
> performance issue and this is an easy way to solve it. IMHO there seems
> to be many ways to solve performance degradation within the driver itself, and
> if those doesn't work then probably it makes sense to revert this.

My initial thought was to revert the change because it was causing a
significant performance regression on certain SoC. But given that it
took over a year for people to follow up, it doesn't seem too urgent, so
we should rather try to understand the issue and potential side effects
of moving back to a 64 byte cache line.

-- 
Catalin