linux-kernel - Re: [PATCH 6/7] tracing/user_events: Use bits vs bytes for enabled status page data

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Date:   Tue, 19 Apr 2022 10:35:45 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Beau Belgrave <beaub@...ux.microsoft.com>
Cc:     rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        linux-trace-devel <linux-trace-devel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH 6/7] tracing/user_events: Use bits vs bytes for enabled
 status page data

----- On Apr 1, 2022, at 7:43 PM, Beau Belgrave beaub@...ux.microsoft.com wrote:

> User processes may require many events and when they do the cache
> performance of a byte index status check is less ideal than a bit index.
> The previous event limit per-page was 4096, the new limit is 32,768.
> 
> This change adds a mask property to the user_reg struct. Programs check
> that the byte at status_index has a bit set by ANDing the status_mask.
> 
> Link:
> https://lore.kernel.org/all/2059213643.196683.1648499088753.JavaMail.zimbra@efficios.com/
> 
> Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Signed-off-by: Beau Belgrave <beaub@...ux.microsoft.com>

Hi Beau,

Considering this will be used in a fast-path, why choose bytewise
loads for the byte at status_index and the status_mask ?

I'm concerned about the performance penalty associated with partial
register stalls when working with bytewise ALU operations rather than
operations using the entire registers.

Ideally I would be tempted to use "unsigned long" type (32-bit on 32-bit
binaries and 64-bit on 64-bit binaries) for both the array access
and the status mask, but this brings extra complexity for 32-bit compat
handling.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com