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

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Date:   Mon, 4 Mar 2019 11:11:41 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Fenghua Yu <fenghua.yu@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, kvm@...r.kernel.org
Subject: Re: [PATCH v4 03/17] wlcore: Align reg_ch_conf_pending and
 tmp_ch_bitmap to unsigned long for better performance

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@...el.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));
>  	struct wiphy *wiphy = wl->hw->wiphy;
>  	struct ieee80211_supported_band *band;
>  	bool timeout = false;
> diff --git a/drivers/net/wireless/ti/wlcore/wlcore.h b/drivers/net/wireless/ti/wlcore/wlcore.h
> index dd14850b0603..92d878f01fa5 100644
> --- a/drivers/net/wireless/ti/wlcore/wlcore.h
> +++ b/drivers/net/wireless/ti/wlcore/wlcore.h
> @@ -321,8 +321,10 @@ struct wl1271 {
>  
>  	/* Reg domain last configuration */
>  	u32 reg_ch_conf_last[2]  __aligned(8);
> -	/* Reg domain pending configuration */
> -	u32 reg_ch_conf_pending[2];
> +	/* Reg domain pending configuration. Aligned to unsigned long for
> +	 * better performane in set_bit().
> +	 */
> +	u32 reg_ch_conf_pending[2] __aligned(sizeof(unsigned long));
>  
>  	/* Pointer that holds DMA-friendly block for the mailbox */
>  	void *mbox;

This has nothing to do with better performance. This is generic code,
not x86 arch code. Many RISC platforms will already refuse unaligned
atomic ops.

Also, how is this set_bit() usage endian safe?

And no wireless person on Cc anywhere.