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 17:26:20 -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 19, 2022, at 2:57 PM, Beau Belgrave beaub@...ux.microsoft.com wrote:

> On Tue, Apr 19, 2022 at 10:35:45AM -0400, Mathieu Desnoyers wrote:
>> ----- 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 ?
>> 
> 
> First, thanks for the review!
> 
> Which loads are you concerned about? The user programs can store the
> index and mask in another type after registration instead of an int.

I'm concerned about the loads from user-space, considering that
those are on the fast-path.

Indeed user programs will need to copy the status index and mask
returned in struct user_reg, so adapting the indexing and mask to
deal with an array of unsigned long rather than bytes can be done
at that point, but I wonder how many users will go through that
extra trouble unless there are helpers to convert the status index
from byte-wise to long-wise, and convert the status mask from a
byte-wise mask to a long-wise mask (and associated documentation).


> 
> However, you may be referring to something on the kernel side?

No.

> 
>> 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.
>> 
> 
> On the kernel side these only occur when a registration happens (pretty
> rare compared to enabled checks) or a delete (even rarer). But I have
> the feeling you are more concerned about the user side, right?

Right.

> 
>> 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.
>> 
> 
> User programs can store the index and mask returned into better value
> types for their architecture.
> 
> I agree it will cause compat handling issues if it's put into the user
> facing header as a long.
> 
> I was hoping APIs, like libtracefs, could abstract many callers from how
> best to use the returned values. For example, it could save the index
> and mask as unsigned long for the callers and use those for the
> enablement checks.
> 
> Do you think there is a way to enable these native types in the ABI
> without causing compat handling issues? I used ints to prevent compat
> issues between 32-bit user mode and 64-bit kernel mode.

I think you are right: this is not an ABI issue, but rather a usability
issue that can be solved by implementing and documenting user-space library
helpers to help user applications index the array and apply the mask to an
unsigned long type.

Thanks,

Mathieu

> 
>> Thanks,
>> 
>> Mathieu
>> 
>> --
>> Mathieu Desnoyers
>> EfficiOS Inc.
>> http://www.efficios.com
> 
> Thanks,
> -Beau

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