Re: [PATCH v4 03/17] wlcore: Align reg_ch_conf_pending and tmp_ch_bitmap to unsigned long for better performance

Paolo Bonzini
2019-03-04T10:46:52Z

On 04/03/19 11:11, Peter Zijlstra wrote:
> On Fri, Mar 01, 2019 at 06:44:57PM -0800, Fenghua Yu wrote:
>> A bit in reg_ch_conf_pending in wl271 and tmp_ch_bitmap is set
>> atomically by set_bit(). set_bit() sets the bit in a single
>> unsigned long location. If the variables are not aligned to
>> unsigned long, set_bit() accesses two cache lines and thus causes
>> slower performance. On x86, this scenario is called split lock and
>> can cause overall performance degradation due to locked BTSL
>> instruction in set_bit() locks bus.
>>
>> To avoid performance degradation, the two variables are aligned to
>> unsigned long.
>>
>> Signed-off-by: Fenghua Yu <fenghua.yu@intel.com> ---
>> drivers/net/wireless/ti/wlcore/cmd.c | 3 ++-
>> drivers/net/wireless/ti/wlcore/wlcore.h | 6 ++++-- 2 files
>> changed, 6 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/net/wireless/ti/wlcore/cmd.c
>> b/drivers/net/wireless/ti/wlcore/cmd.c index
>> 903968735a74..8d15a6307d44 100644 ---
>> a/drivers/net/wireless/ti/wlcore/cmd.c +++
>> b/drivers/net/wireless/ti/wlcore/cmd.c @@ -1707,7 +1707,8 @@ int
>> wlcore_cmd_regdomain_config_locked(struct wl1271 *wl) { struct
>> wl12xx_cmd_regdomain_dfs_config *cmd = NULL; int ret = 0, i, b,
>> ch_bit_idx; - u32 tmp_ch_bitmap[2]; + /* Align to unsigned long
>> for better performance in set_bit() */ + u32 tmp_ch_bitmap[2]
>> __aligned(sizeof(unsigned long));

This is the only place where an array of u32 is needed, because of

cmd->ch_bit_map1 = cpu_to_le32(tmp_ch_bitmap[0]);
cmd->ch_bit_map2 = cpu_to_le32(tmp_ch_bitmap[1]);

All the others should use DECLARE_BITMAP, including reg_ch_conf_last
which was already using __aligned. As Peter mentioned they should
also use set_bit_le. Actually they do not need locked access at all
because the only code paths to the set_bit take a mutex.

There is one other place that is accessing the items of the array, but
it is fixed easily. The following patch should do everything:

------------------- 8< --------------------------
From: Paolo Bonzini <pbonzini@redhat.com>
Subject: [PATCH] wlcore: simplify/fix/optimize reg_ch_conf_pending operations

Bitmaps are defined on unsigned longs, so the usage of u32[2] in the
wlcore driver is incorrect. As noted by Peter Zijlstra, casting arrays
to a bitmap is incorrect for big-endian architectures.

When looking at it I observed that:

- operations on reg_ch_conf_pending is always under the wl_lock mutex,
so set_bit is overkill

- the only case where reg_ch_conf_pending is accessed a u32 at a time is
unnecessary too.

This patch cleans up everything in this area, and changes tmp_ch_bitmap
to have the proper alignment.

Reported-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/drivers/net/wireless/ti/wlcore/cmd.c b/drivers/net/wireless/ti/wlcore/cmd.c
index 903968735a74..3e093f3a7ec8 100644
--- a/drivers/net/wireless/ti/wlcore/cmd.c
+++ b/drivers/net/wireless/ti/wlcore/cmd.c
@@ -1700,14 +1700,14 @@ void wlcore_set_pending_regdomain_ch(struct wl1271 *wl, u16 channel,
ch_bit_idx = wlcore_get_reg_conf_ch_idx(band, channel);

if (ch_bit_idx >= 0 && ch_bit_idx <= WL1271_MAX_CHANNELS)
- set_bit(ch_bit_idx, (long *)wl->reg_ch_conf_pending);
+ __set_bit_le(ch_bit_idx, (long *)wl->reg_ch_conf_pending);
}

int wlcore_cmd_regdomain_config_locked(struct wl1271 *wl)
{
struct wl12xx_cmd_regdomain_dfs_config *cmd = NULL;
int ret = 0, i, b, ch_bit_idx;
- u32 tmp_ch_bitmap[2];
+ u32 tmp_ch_bitmap[2] __aligned(sizeof(unsigned long));
struct wiphy *wiphy = wl->hw->wiphy;
struct ieee80211_supported_band *band;
bool timeout = false;
@@ -1717,7 +1717,7 @@ int wlcore_cmd_regdomain_config_locked(struct wl1271 *wl)

wl1271_debug(DEBUG_CMD, "cmd reg domain config");

- memset(tmp_ch_bitmap, 0, sizeof(tmp_ch_bitmap));
+ memcpy(tmp_ch_bitmap, wl->reg_ch_conf_pending, sizeof(tmp_ch_bitmap));

for (b = NL80211_BAND_2GHZ; b <= NL80211_BAND_5GHZ; b++) {
band = wiphy->bands[b];
@@ -1738,13 +1738,10 @@ int wlcore_cmd_regdomain_config_locked(struct wl1271 *wl)
if (ch_bit_idx < 0)
continue;

- set_bit(ch_bit_idx, (long *)tmp_ch_bitmap);
+ __set_bit_le(ch_bit_idx, (long *)tmp_ch_bitmap);
}
}

- tmp_ch_bitmap[0] |= wl->reg_ch_conf_pending[0];
- tmp_ch_bitmap[1] |= wl->reg_ch_conf_pending[1];
-
if (!memcmp(tmp_ch_bitmap, wl->reg_ch_conf_last, sizeof(tmp_ch_bitmap)))
goto out;

diff --git a/drivers/net/wireless/ti/wlcore/wlcore.h b/drivers/net/wireless/ti/wlcore/wlcore.h
index dd14850b0603..870eea3e7a27 100644
--- a/drivers/net/wireless/ti/wlcore/wlcore.h
+++ b/drivers/net/wireless/ti/wlcore/wlcore.h
@@ -320,9 +320,9 @@ struct wl1271 {
bool watchdog_recovery;

/* Reg domain last configuration */
- u32 reg_ch_conf_last[2] __aligned(8);
+ DECLARE_BITMAP(reg_ch_conf_last, 64);
/* Reg domain pending configuration */
- u32 reg_ch_conf_pending[2];
+ DECLARE_BITMAP(reg_ch_conf_pending, 64);

/* Pointer that holds DMA-friendly block for the mailbox */
void *mbox;