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authorAllen Hubbe <Allen.Hubbe@emc.com>2015-07-13 15:07:21 +0300
committerJon Mason <jdmason@kudzu.us>2015-09-07 22:17:08 +0300
commit905921e74864e80228e7f8cfe75315cd0a8cada8 (patch)
treea9d9d5d567bbfeef421e9610296ed7a55b15caf8 /drivers
parentd98ef99e378b0d5c42be928d6f2abe08a5f9ce53 (diff)
downloadlinux-905921e74864e80228e7f8cfe75315cd0a8cada8.tar.xz
NTB: Remove dma_sync_wait from ntb_async_rx
The dma_sync_wait can hurt the performance of workloads mixed with both large and small frames. Large frames will be copied using the dma engine. Small frames will be copied by the cpu. The dma_sync_wait prevents the cpu and dma engine copying in parallel. In the period where the cpu is copying, the dma engine is stopped. The dma engine is not doing any useful work to copy large frames during that time, and the additional time to restart the dma engine for the next large frame. This will decrease the throughput for the portion of a workload with large frames. In the period where the dma engine is copying, the cpu is held up waiting for dma to complete. The small frames processing will be delayed until the dma is complete. The RX frames are completed in-order, and the processing of small frames takes very little time, so dma_sync_wait may have an insignificant impact on the respose time of frames. The more significant impact is to the system, because the delay in dma_sync_wait is implemented as busy non-blocking wait. This can prevent the delayed core from doing any useful work, even if it could be processing work for other drivers, unrelated to transport RX processing. After applying the earlier patch to fix out-of-order RX acknoledgement, the dma_sync_wait is no longer necessary. Remove it, so that cpu memcpy will proceed immediately for small frames, in parallel with ongoing dma for large frames. Do not hold up the cpu from doing work while dma is in progress. The prior fix will continue to ensure in-order completion of the RX frames to the upper layer, and in-order delivery of the RX acknoledgement. Signed-off-by: Allen Hubbe <Allen.Hubbe@emc.com> Signed-off-by: Jon Mason <jdmason@kudzu.us>
Diffstat (limited to 'drivers')
-rw-r--r--drivers/ntb/ntb_transport.c12
1 files changed, 3 insertions, 9 deletions
diff --git a/drivers/ntb/ntb_transport.c b/drivers/ntb/ntb_transport.c
index 777436c47679..f6aae0fbde48 100644
--- a/drivers/ntb/ntb_transport.c
+++ b/drivers/ntb/ntb_transport.c
@@ -1233,18 +1233,18 @@ static void ntb_async_rx(struct ntb_queue_entry *entry, void *offset)
goto err;
if (len < copy_bytes)
- goto err_wait;
+ goto err;
device = chan->device;
pay_off = (size_t)offset & ~PAGE_MASK;
buff_off = (size_t)buf & ~PAGE_MASK;
if (!is_dma_copy_aligned(device, pay_off, buff_off, len))
- goto err_wait;
+ goto err;
unmap = dmaengine_get_unmap_data(device->dev, 2, GFP_NOWAIT);
if (!unmap)
- goto err_wait;
+ goto err;
unmap->len = len;
unmap->addr[0] = dma_map_page(device->dev, virt_to_page(offset),
@@ -1287,12 +1287,6 @@ err_set_unmap:
dmaengine_unmap_put(unmap);
err_get_unmap:
dmaengine_unmap_put(unmap);
-err_wait:
- /* If the callbacks come out of order, the writing of the index to the
- * last completed will be out of order. This may result in the
- * receive stalling forever.
- */
- dma_sync_wait(chan, qp->last_cookie);
err:
ntb_memcpy_rx(entry, offset);
qp->rx_memcpy++;