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Message-ID: <a94acd88-dab9-048d-ed9b-b4cac6c4d794@arm.com>
Date: Wed, 1 Sep 2021 21:03:30 +0100
From: Robin Murphy <robin.murphy@....com>
To: Leo Yan <leo.yan@...aro.org>,
Mathieu Poirier <mathieu.poirier@...aro.org>,
Suzuki K Poulose <suzuki.poulose@....com>,
Mike Leach <mike.leach@...aro.org>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
coresight@...ts.linaro.org, linux-arm-kernel@...ts.infradead.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] coresight: tmc-etr: Speed up for bounce buffer in flat
mode
On 2021-08-29 14:54, Leo Yan wrote:
> The AUX bounce buffer is allocated with API dma_alloc_coherent(), in the
> low level's architecture code, e.g. for Arm64, it maps the memory with
> the attribution "Normal non-cacheable"; this can be concluded from the
> definition for pgprot_dmacoherent() in arch/arm64/include/asm/pgtable.h.
>
> Later when access the AUX bounce buffer, since the memory mapping is
> non-cacheable, it's low efficiency due to every load instruction must
> reach out DRAM.
>
> This patch changes to allocate pages with alloc_pages_node(), thus the
> driver can access the memory with cacheable mapping in the kernel linear
> virtual address; therefore, because load instructions can fetch data
> from cache lines rather than always read data from DRAM, the driver can
> boost memory coping performance. After using the cacheable mapping, the
> driver uses dma_sync_single_for_cpu() to invalidate cacheline prior to
> read bounce buffer so can avoid read stale trace data.
>
> By measurement the duration for function tmc_update_etr_buffer() with
> ftrace function_graph tracer, it shows the performance significant
> improvement for copying 4MiB data from bounce buffer:
>
> # echo tmc_etr_get_data_flat_buf > set_graph_notrace // avoid noise
> # echo tmc_update_etr_buffer > set_graph_function
> # echo function_graph > current_tracer
>
> before:
>
> # CPU DURATION FUNCTION CALLS
> # | | | | | | |
> 2) | tmc_update_etr_buffer() {
> ...
> 2) # 8148.320 us | }
>
> after:
>
> # CPU DURATION FUNCTION CALLS
> # | | | | | | |
> 2) | tmc_update_etr_buffer() {
> ...
> 2) # 2463.980 us | }
>
> Signed-off-by: Leo Yan <leo.yan@...aro.org>
> Reviewed-by: Suzuki K Poulose <suzuki.poulose@....com>
> ---
>
> Changes from v2:
> Sync the entire buffer in one go when the tracing is wrap around
> (Suzuki);
> Add Suzuki's review tage.
>
> Changes from v1:
> Set "flat_buf->daddr" to 0 when fails to map DMA region; and dropped the
> unexpected if condition change in tmc_etr_free_flat_buf().
>
> .../hwtracing/coresight/coresight-tmc-etr.c | 47 ++++++++++++++++---
> 1 file changed, 40 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/hwtracing/coresight/coresight-tmc-etr.c b/drivers/hwtracing/coresight/coresight-tmc-etr.c
> index 13fd1fc730ed..ac37e9376d2b 100644
> --- a/drivers/hwtracing/coresight/coresight-tmc-etr.c
> +++ b/drivers/hwtracing/coresight/coresight-tmc-etr.c
> @@ -21,6 +21,7 @@
>
> struct etr_flat_buf {
> struct device *dev;
> + struct page *pages;
> dma_addr_t daddr;
> void *vaddr;
> size_t size;
> @@ -600,6 +601,7 @@ static int tmc_etr_alloc_flat_buf(struct tmc_drvdata *drvdata,
> {
> struct etr_flat_buf *flat_buf;
> struct device *real_dev = drvdata->csdev->dev.parent;
> + ssize_t aligned_size;
>
> /* We cannot reuse existing pages for flat buf */
> if (pages)
> @@ -609,11 +611,18 @@ static int tmc_etr_alloc_flat_buf(struct tmc_drvdata *drvdata,
> if (!flat_buf)
> return -ENOMEM;
>
> - flat_buf->vaddr = dma_alloc_coherent(real_dev, etr_buf->size,
> - &flat_buf->daddr, GFP_KERNEL);
> - if (!flat_buf->vaddr) {
> - kfree(flat_buf);
> - return -ENOMEM;
> + aligned_size = PAGE_ALIGN(etr_buf->size);
> + flat_buf->pages = alloc_pages_node(node, GFP_KERNEL | __GFP_ZERO,
> + get_order(aligned_size));
> + if (!flat_buf->pages)
> + goto fail_alloc_pages;
> +
> + flat_buf->vaddr = page_address(flat_buf->pages);
> + flat_buf->daddr = dma_map_page(real_dev, flat_buf->pages, 0,
> + aligned_size, DMA_FROM_DEVICE);
Use dma_alloc_noncoherent() rather than open-coding this - bare
alloc_pages() has no understanding of DMA masks, and you wouldn't want
to end up in the worst case of dma_map_page() bounce-buffering your
bounce buffer...
Robin.
> + if (dma_mapping_error(real_dev, flat_buf->daddr)) {
> + flat_buf->daddr = 0;
> + goto fail_dma_map_page;
> }
>
> flat_buf->size = etr_buf->size;
> @@ -622,6 +631,12 @@ static int tmc_etr_alloc_flat_buf(struct tmc_drvdata *drvdata,
> etr_buf->mode = ETR_MODE_FLAT;
> etr_buf->private = flat_buf;
> return 0;
> +
> +fail_dma_map_page:
> + __free_pages(flat_buf->pages, get_order(aligned_size));
> +fail_alloc_pages:
> + kfree(flat_buf);
> + return -ENOMEM;
> }
>
> static void tmc_etr_free_flat_buf(struct etr_buf *etr_buf)
> @@ -630,15 +645,20 @@ static void tmc_etr_free_flat_buf(struct etr_buf *etr_buf)
>
> if (flat_buf && flat_buf->daddr) {
> struct device *real_dev = flat_buf->dev->parent;
> + ssize_t aligned_size = PAGE_ALIGN(etr_buf->size);
>
> - dma_free_coherent(real_dev, flat_buf->size,
> - flat_buf->vaddr, flat_buf->daddr);
> + dma_unmap_page(real_dev, flat_buf->daddr, aligned_size,
> + DMA_FROM_DEVICE);
> + __free_pages(flat_buf->pages, get_order(aligned_size));
> }
> kfree(flat_buf);
> }
>
> static void tmc_etr_sync_flat_buf(struct etr_buf *etr_buf, u64 rrp, u64 rwp)
> {
> + struct etr_flat_buf *flat_buf = etr_buf->private;
> + struct device *real_dev = flat_buf->dev->parent;
> +
> /*
> * Adjust the buffer to point to the beginning of the trace data
> * and update the available trace data.
> @@ -648,6 +668,19 @@ static void tmc_etr_sync_flat_buf(struct etr_buf *etr_buf, u64 rrp, u64 rwp)
> etr_buf->len = etr_buf->size;
> else
> etr_buf->len = rwp - rrp;
> +
> + /*
> + * The driver always starts tracing at the beginning of the buffer,
> + * the only reason why we would get a wrap around is when the buffer
> + * is full. Sync the entire buffer in one go for this case.
> + */
> + if (etr_buf->offset + etr_buf->len > etr_buf->size)
> + dma_sync_single_for_cpu(real_dev, flat_buf->daddr,
> + etr_buf->size, DMA_FROM_DEVICE);
> + else
> + dma_sync_single_for_cpu(real_dev,
> + flat_buf->daddr + etr_buf->offset,
> + etr_buf->len, DMA_FROM_DEVICE);
> }
>
> static ssize_t tmc_etr_get_data_flat_buf(struct etr_buf *etr_buf,
>
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