From: Roland Dreier <Roland Dreier roland@xxxxxxxxxxxxxxx>
Doing alloc_page(GFP_KERNEL | __GFP_ZERO) to get pages used for data
buffers wastes a lot of CPU clearing pages that will be quickly be
overwritten by the actual data. However, for emulated control
commands such as INQUIRY and so on, the code does assume that the
buffer is zeroed.
To avoid this CPU overhead, skip the __GFP_ZERO for commands that are
actually moving data, ie cmds that have SCF_SCSI_DATA_SG_IO_CDB set.
Signed-off-by: Roland Dreier <Roland Dreier roland@xxxxxxxxxxxxxxx>
---
I'm not totally sure this is correct so please double-check my
reasoning, but it is certainly true that clear_page shows up high
on profiles running high throughput workloads, and there doesn't
seem to be any good reason for that.
drivers/target/target_core_transport.c | 5 ++++-
1 files changed, 4 insertions(+), 1 deletions(-)
diff --git a/drivers/target/target_core_transport.c b/drivers/target/target_core_transport.c
index 0257658..419ffd6 100644
--- a/drivers/target/target_core_transport.c
+++ b/drivers/target/target_core_transport.c
@@ -3461,6 +3461,7 @@ transport_generic_get_mem(struct se_cmd *cmd)
u32 length = cmd->data_length;
unsigned int nents;
struct page *page;
+ gfp_t zero_flag;
int i = 0;
nents = DIV_ROUND_UP(length, PAGE_SIZE);
@@ -3471,9 +3472,11 @@ transport_generic_get_mem(struct se_cmd *cmd)
cmd->t_data_nents = nents;
sg_init_table(cmd->t_data_sg, nents);
+ zero_flag = cmd->se_cmd_flags & SCF_SCSI_DATA_SG_IO_CDB ? 0 : __GFP_ZERO;
+
while (length) {
u32 page_len = min_t(u32, length, PAGE_SIZE);
- page = alloc_page(GFP_KERNEL | __GFP_ZERO);
+ page = alloc_page(GFP_KERNEL | zero_flag);
if (!page)
goto out;
--
1.7.7.3
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