[PATCH v2 1/1] net/mlx5: Added cond_resched() to crdump collection

[Mohamed Khalfella <mkhalfella@purestorage.com>]

Collecting crdump involves reading vsc registers from pci config space
of mlx device, which can take long time to complete. This might result
in starving other threads waiting to run on the cpu.

Numbers I got from testing ConnectX-5 Ex MCX516A-CDAT in the lab:

- mlx5_vsc_gw_read_block_fast() was called with length = 1310716.
- mlx5_vsc_gw_read_fast() reads 4 bytes at a time. It was not used to
  read the entire 1310716 bytes. It was called 53813 times because
  there are jumps in read_addr.
- On average mlx5_vsc_gw_read_fast() took 35284.4ns.
- In total mlx5_vsc_wait_on_flag() called vsc_read() 54707 times.
  The average time for each call was 17548.3ns. In some instances
  vsc_read() was called more than one time when the flag was not set.
  As expected the thread released the cpu after 16 iterations in
  mlx5_vsc_wait_on_flag().
- Total time to read crdump was 35284.4ns * 53813 ~= 1.898s.

It was seen in the field that crdump can take more than 5 seconds to
complete. During that time mlx5_vsc_wait_on_flag() did not release the
cpu because it did not complete 16 iterations. It is believed that pci
config reads were slow. This change adds conditional reschedule call
every 128 register read to release the cpu if needed.

Reviewed-by: Yuanyuan Zhong <yzhong@purestorage.com>
Signed-off-by: Mohamed Khalfella <mkhalfella@purestorage.com>
---
 drivers/net/ethernet/mellanox/mlx5/core/lib/pci_vsc.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/lib/pci_vsc.c b/drivers/net/ethernet/mellanox/mlx5/core/lib/pci_vsc.c
index 6b774e0c2766..bc6c38a68702 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/lib/pci_vsc.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/lib/pci_vsc.c
@@ -269,6 +269,7 @@ int mlx5_vsc_gw_read_block_fast(struct mlx5_core_dev *dev, u32 *data,
 {
 	unsigned int next_read_addr = 0;
 	unsigned int read_addr = 0;
+	unsigned int count = 0;
 
 	while (read_addr < length) {
 		if (mlx5_vsc_gw_read_fast(dev, read_addr, &next_read_addr,
@@ -276,6 +277,9 @@ int mlx5_vsc_gw_read_block_fast(struct mlx5_core_dev *dev, u32 *data,
 			return read_addr;
 
 		read_addr = next_read_addr;
+		/* Yield the cpu every 128 register read */
+		if ((++count & 0x7f) == 0)
+			cond_resched();
 	}
 	return length;
 }
-- 
2.45.2