From: Lorenzo Bianconi <lorenzo@kernel.org>
To: Jesper Dangaard Brouer <brouer@redhat.com>
Cc: netdev@vger.kernel.org, davem@davemloft.net,
ilias.apalodimas@linaro.org, lorenzo.bianconi@redhat.com,
mcroce@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 16:17:23 +0200 [thread overview]
Message-ID: <20191116141723.GG20820@localhost.localdomain> (raw)
In-Reply-To: <20191116131741.2657e1bb@carbon>
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> 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.
right, I can see it now :)
>
>
> > > > @@ -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)
I guess we can move page_pool_dma_sync_for_device() before
__page_pool_recycle_direct and __page_pool_recycle_into_ring since even if
__page_pool_put_page is not running in NAPI context or if alloc.cache is full
we will end up calling page_pool_dma_sync_for_device as first task in
__page_pool_recycle_into_ring. I will fix in v4.
Regards,
Lorenzo
>
> --
> Best regards,
> Jesper Dangaard Brouer
> MSc.CS, Principal Kernel Engineer at Red Hat
> LinkedIn: http://www.linkedin.com/in/brouer
>
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next prev parent reply other threads:[~2019-11-16 14:17 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
2019-11-16 14:17 ` Lorenzo Bianconi [this message]
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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