linux-kernel - Re: [PATCH v3 09/15] arm64/mm: Split the function check_and_switch

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Message-ID: <20190725162827.GE4113@arrakis.emea.arm.com>
Date:   Thu, 25 Jul 2019 17:28:28 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Julien Grall <julien.grall@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.cs.columbia.edu, james.morse@....com,
        marc.zyngier@....com, julien.thierry@....com,
        suzuki.poulose@....com, will.deacon@....com
Subject: Re: [PATCH v3 09/15] arm64/mm: Split the function
 check_and_switch_context in 3 parts

On Wed, Jul 24, 2019 at 05:25:28PM +0100, Julien Grall wrote:
> The function check_and_switch_context is used to:
>     1) Check whether the ASID is still valid
>     2) Generate a new one if it is not valid
>     3) Switch the context
> 
> While the latter is specific to the MM subsystem, the rest could be part
> of the generic ASID allocator.
> 
> After this patch, the function is now split in 3 parts which corresponds
> to the use of the functions:
>     1) asid_check_context: Check if the ASID is still valid
>     2) asid_new_context: Generate a new ASID for the context
>     3) check_and_switch_context: Call 1) and 2) and switch the context
> 
> 1) and 2) have not been merged in a single function because we want to
> avoid to add a branch in when the ASID is still valid. This will matter
> when the code will be moved in separate file later on as 1) will reside
> in the header as a static inline function.
> 
> Signed-off-by: Julien Grall <julien.grall@....com>
> 
> ---
>     Will wants to avoid to add a branch when the ASID is still valid. So
>     1) and 2) are in separates function. The former will move to a new
>     header and make static inline.

Was this discussion logged somewhere, just to get the context?

I presume by "branch" you meant the function call to
asid_check_context(). Personally, I don't like the duplication of this
function in patch 13. This is part of the ASID allocation algorithm and
I prefer to keep them together (we even had a bug in here with the xchg
use).

Do you have any numbers to show how non-inlining this function affects
the performance (hackbench -P would do).

-- 
Catalin