On Thu, 24 Nov 2016 10:00:36 +0100
Arnd Bergmann <a...@arndb.de> wrote:
> On Thursday, November 24, 2016 4:37:36 PM CET Jisheng Zhang wrote:
> > solB (a SW shadow cookie) perhaps gives a better performance: in hot path,
> > such as mvneta_rx(), the driver accesses buf_cookie and buf_phys_addr of
> > rx_desc which is allocated by dma_alloc_coherent, it's noncacheable if the
> > device isn't cache-coherent. I didn't measure the performance difference,
> > because in fact we take solA as well internally. From your experience,
> > can the performance gain deserve the complex code?
>
> Yes, a read from uncached memory is fairly slow, so if you have a chance
> to avoid that it will probably help. When adding complexity to the code,
> it probably makes sense to take a runtime profile anyway quantify how
> much it gains.
>
> On machines that have cache-coherent DMA, accessing the descriptor
> should be fine, as you already have to load the entire cache line
> to read the status field.
>
> Looking at this snippet:
>
> rx_status = rx_desc->status;
> rx_bytes = rx_desc->data_size - (ETH_FCS_LEN +
> MVNETA_MH_SIZE);
> data = (unsigned char *)rx_desc->buf_cookie;
> phys_addr = rx_desc->buf_phys_addr;
> pool_id = MVNETA_RX_GET_BM_POOL_ID(rx_desc);
> bm_pool = &pp->bm_priv->bm_pools[pool_id];
>
> if (!mvneta_rxq_desc_is_first_last(rx_status) ||
> (rx_status & MVNETA_RXD_ERR_SUMMARY)) {
> err_drop_frame_ret_pool:
> /* Return the buffer to the pool */
> mvneta_bm_pool_put_bp(pp->bm_priv, bm_pool,
> rx_desc->buf_phys_addr);
> err_drop_frame:
>
>
> I think there is more room for optimizing if you start: you read
> the status field twice (the second one in MVNETA_RX_GET_BM_POOL_ID)
> and you can cache the buf_phys_addr along with the virtual address
> once you add that.
oh, yeah! buf_phy_addr could be included too.
>
> Generally speaking, I'd recommend using READ_ONCE()/WRITE_ONCE()
> to access the descriptor fields, to ensure the compiler doesn't
> add extra references as well as to annotate the expensive
> operations.
>
> Arnd
Got it. Thanks so much for the detailed guide.
