Re: [PATCH net-next 0/6] page_pool: recycle buffers

Ilias Apalodimas
2021-03-23T17:02:51Z

On Tue, Mar 23, 2021 at 04:55:31PM +0000, Alexander Lobakin wrote:
> > > > > >

[...]

> > > > >
> > > > > Thanks for the testing!
> > > > > Any chance you can get a perf measurement on this?
> > > >
> > > > I guess you mean perf-report (--stdio) output, right?
> > > >
> > >
> > > Yea,
> > > As hinted below, I am just trying to figure out if on Alexander's platform the
> > > cost of syncing, is bigger that free-allocate. I remember one armv7 were that
> > > was the case.
> > >
> > > > > Is DMA syncing taking a substantial amount of your cpu usage?
> > > >
> > > > (+1 this is an important question)
>
> Sure, I'll drop perf tools to my test env and share the results,
> maybe tomorrow or in a few days.
> From what I know for sure about MIPS and my platform,
> post-Rx synching (dma_sync_single_for_cpu()) is a no-op, and
> pre-Rx (dma_sync_single_for_device() etc.) is a bit expensive.
> I always have sane page_pool->pp.max_len value (smth about 1668
> for MTU of 1500) to minimize the overhead.
>
> By the word, IIRC, all machines shipped with mvpp2 have hardware
> cache coherency units and don't suffer from sync routines at all.
> That may be the reason why mvpp2 wins the most from this series.

Yep exactly. It's also the reason why you explicitly have to opt-in using the
recycling (by marking the skb for it), instead of hiding the feature in the
page pool internals

Cheers
/Ilias

>
> > > > > >
> > > > > > [0] https://lore.kernel.org/netdev/20210323153550.130385-1-alobakin@pm.me
> > > > > >
> > > >
> >
> > That would be the same as for mvneta:
> >
> > Overhead Shared Object Symbol
> > 24.10% [kernel] [k] __pi___inval_dcache_area
> > 23.02% [mvneta] [k] mvneta_rx_swbm
> > 7.19% [kernel] [k] kmem_cache_alloc
> >
> > Anyway, I tried to use the recycling *and* napi_build_skb on mvpp2,
> > and I get lower packet rate than recycling alone.
> > I don't know why, we should investigate it.
>
> mvpp2 driver doesn't use napi_consume_skb() on its Tx completion path.
> As a result, NAPI percpu caches get refilled only through
> kmem_cache_alloc_bulk(), and most of skbuff_head recycling
> doesn't work.
>
> > Regards,
> > --
> > per aspera ad upstream
>
> Oh, I love that one!
>
> Al
>