From: Jesper Dangaard Brouer <brouer@redhat.com>
To: Lorenzo Bianconi <lorenzo@kernel.org>
Cc: netdev@vger.kernel.org, davem@davemloft.net,
ilias.apalodimas@linaro.org, lorenzo.bianconi@redhat.com,
mcroce@redhat.com, brouer@redhat.com
Subject: Re: [PATCH v3 net-next 2/3] net: page_pool: add the possibility to sync DMA memory for device
Date: Sat, 16 Nov 2019 13:17:41 +0100 [thread overview]
Message-ID: <20191116131741.2657e1bb@carbon> (raw)
In-Reply-To: <20191116113630.GB20820@localhost.localdomain>
On Sat, 16 Nov 2019 13:36:30 +0200
Lorenzo Bianconi <lorenzo@kernel.org> wrote:
> > On Fri, 15 Nov 2019 21:01:38 +0200
> > Lorenzo Bianconi <lorenzo@kernel.org> wrote:
> >
> > > static bool __page_pool_recycle_into_ring(struct page_pool *pool,
> > > - struct page *page)
> > > + struct page *page,
> > > + unsigned int dma_sync_size)
> > > {
> > > int ret;
> > > /* BH protection not needed if current is serving softirq */
> > > @@ -264,6 +285,9 @@ static bool __page_pool_recycle_into_ring(struct page_pool *pool,
> > > else
> > > ret = ptr_ring_produce_bh(&pool->ring, page);
> > >
> > > + if (ret == 0 && (pool->p.flags & PP_FLAG_DMA_SYNC_DEV))
> > > + page_pool_dma_sync_for_device(pool, page, dma_sync_size);
> > > +
> > > return (ret == 0) ? true : false;
> > > }
> >
> >
> > I do wonder if we should DMA-sync-for-device BEFORE putting page into
> > ptr_ring, as this is a channel between several concurrent CPUs.
>
> Hi Jesper,
>
> in this way we can end up syncing the DMA page even if it is unmapped in
> __page_pool_clean_page (e.g. if the ptr_ring is full), right?
Yes. The call __page_pool_clean_page() will do a dma_unmap_page, so it
should still be safe/correct. I can see, that it is not optimal
performance wise, in-case the ptr_ring is full, as DMA-sync-for-device
is wasted work.
I don't know if you can find an argument, that proves that it cannot
happen, that a remote CPU can dequeue/consume the page from ptr_ring
and give it to the device, while you (the CPU the enqueued) are still
doing the DMA-sync-for-device.
> > > @@ -273,18 +297,22 @@ static bool __page_pool_recycle_into_ring(struct page_pool *pool,
> > > * Caller must provide appropriate safe context.
> > > */
> > > static bool __page_pool_recycle_direct(struct page *page,
> > > - struct page_pool *pool)
> > > + struct page_pool *pool,
> > > + unsigned int dma_sync_size)
> > > {
> > > if (unlikely(pool->alloc.count == PP_ALLOC_CACHE_SIZE))
> > > return false;
> > >
> > > /* Caller MUST have verified/know (page_ref_count(page) == 1) */
> > > pool->alloc.cache[pool->alloc.count++] = page;
> > > +
> > > + if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV)
> > > + page_pool_dma_sync_for_device(pool, page, dma_sync_size);
> > > return true;
> > > }
> >
> > We know __page_pool_recycle_direct() is concurrency safe, and only a
> > single (NAPI processing) CPU can enter. (So, the DMA-sync order is not
> > wrong here, but it could be swapped)
>
> do you mean move it before putting the page in the cache?
>
> pool->alloc.cache[pool->alloc.count++] = page;
Yes, but here the order doesn't matter.
If you choose to do the DMA-sync-for-device earlier/before, then look
at the code, and see of it makes sense to do it in __page_pool_put_page() ?
(I've not checked the details)
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
next prev parent reply other threads:[~2019-11-16 12:17 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-15 19:01 [PATCH v3 net-next 0/3] add DMA-sync-for-device capability to page_pool API Lorenzo Bianconi
2019-11-15 19:01 ` [PATCH v3 net-next 1/3] net: mvneta: rely on page_pool_recycle_direct in mvneta_run_xdp Lorenzo Bianconi
2019-11-15 19:01 ` [PATCH v3 net-next 2/3] net: page_pool: add the possibility to sync DMA memory for device Lorenzo Bianconi
2019-11-15 19:35 ` Jonathan Lemon
2019-11-16 11:20 ` Jesper Dangaard Brouer
2019-11-16 11:36 ` Lorenzo Bianconi
2019-11-16 12:17 ` Jesper Dangaard Brouer [this message]
2019-11-16 14:17 ` Lorenzo Bianconi
2019-11-15 19:01 ` [PATCH v3 net-next 3/3] net: mvneta: get rid of huge dma sync in mvneta_rx_refill Lorenzo Bianconi
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