linux-kernel - Re: [PATCH v2 2/4] platform/mellanox: mlxbf-pmc: Add support for 64-bit counters and cycle count

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Date: Tue, 11 Jun 2024 10:14:59 +0300 (EEST)
From: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To: Shravan Ramani <shravankr@...dia.com>
cc: Hans de Goede <hdegoede@...hat.com>, Vadim Pasternak <vadimp@...dia.com>, 
    David Thompson <davthompson@...dia.com>, 
    "platform-driver-x86@...r.kernel.org" <platform-driver-x86@...r.kernel.org>, 
    LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/4] platform/mellanox: mlxbf-pmc: Add support for
 64-bit counters and cycle count

On Mon, 3 Jun 2024, Shravan Ramani wrote:

> > > Both these features are supported by BlueField-3 PMC hardware, hence
> > > the required bit-fields are exposed by the driver via sysfs to allow
> > > the user to configure as needed.
> > 
> > I'm trying to understand what happens for the other counter, when the
> > use_odd_counter is enabled? This change also doesn't add code that would
> > make the other counter -EBUSY, should that be done?
> 
> When 2 32-bit counters are coupled to form a 64-bit counter using this setting,
> one counter will hold the lower 32 bits while the other will hold the upper 32.
> So the other counter (or syses corresponding to it) also needs to be accessed.
> 
> > For 64-bit counter, I suppose the userspace is expected to read the full
> > counter from two sysfs files and combine the value (your documentation
> > doesn't explain this)? That seems non-optimal, why cannot kernel just
> > return the full combined 64-value directly in kernel?
> 
> I will add more clear comments for this.
> While it is true that the driver could combine the 2 fields and present a
> 64-bit value via one of the sysfs, the reason for the current approach is that
> there are other interfaces which expose the same counters for our platform
> and there are tools that are expected to work on top of both interfaces for
> the purpose of collecting performance stats.

> The other interfaces follow this
> approach of having lower and upper 32-bits separately in each counter, and
> the tools expect the same. Hence the driver follows this approach to keep
> things consistent across the BlueField platform.

Hi,

I went to look through the existing arrays in mlxbf-pmc.c but did not find 
any entries that would have clearly indicated the counters being hi/lo 
parts of the same counter. There were a few 0/1 ones which could be the 
same counter although I suspect even they are not parts of the same 
counter but two separate entities called 0 and 1 having the same counter.

Could you please elaborate further what you meant with the note about 
other interfaces above so I can better assess the claim?

-- 
 i.