From: Will Deacon <will@kernel.org>
To: Guo Hui <guohui@uniontech.com>
Cc: catalin.marinas@arm.com, linux-arm-kernel@lists.infradead.org,
quic_jiles@quicinc.com, mark.rutland@arm.com,
wangxiaohua@uniontech.com
Subject: Re: [PATCH] arm64: mm: Optimize querying asid from reserved_asids
Date: Fri, 29 Sep 2023 17:51:21 +0100 [thread overview]
Message-ID: <20230929165119.GA30869@willie-the-truck> (raw)
In-Reply-To: <20230927055106.14635-1-guohui@uniontech.com>
On Wed, Sep 27, 2023 at 01:51:06PM +0800, Guo Hui wrote:
> Move reserved_asids updates into function flush_context. When
> asid_generation increases, reserved_asids are updated synchronously.
> The execution frequency of function flush_context is far less than
> that of function check_update_reserved_asid. In function
> check_update_reserved_asid, you only need to query whether it is
> in reserved_asids based on the new newasid, and there is no need
> to update it.
>
> In the function check_update_reserved_asid, among all the times
> reserved_asids are hit, the probability that newasid is equal to
> the reserved_asids of the current CPU is about greater than 70%.
Do you have any performance numbers to justify this change?
> diff --git a/arch/arm64/mm/context.c b/arch/arm64/mm/context.c
> index 188197590fc9..76e0beb14466 100644
> --- a/arch/arm64/mm/context.c
> +++ b/arch/arm64/mm/context.c
> @@ -101,7 +101,7 @@ static void set_reserved_asid_bits(void)
> #define asid_gen_match(asid) \
> (!(((asid) ^ atomic64_read(&asid_generation)) >> asid_bits))
>
> -static void flush_context(void)
> +static void flush_context(u64 generation)
> {
> int i;
> u64 asid;
> @@ -120,6 +120,8 @@ static void flush_context(void)
> */
> if (asid == 0)
> asid = per_cpu(reserved_asids, i);
> +
> + asid = generation | (asid & ~ASID_MASK);
I'm struggling to see how this is safe with multiple rollovers. For
example, if we have the following sequence of events on one CPU:
1. Task A runs with ASID 42
2. Context switch to Task C with ASID 66
3. A rollover occurs
4. Task B runs for the first time; gets allocated ASID 42
5. Another rollover occurs
6. Task A runs
Won't task A get ASID 42, even though that's reserved for task B?
But this code is subtle, so maybe I missed something.
Also, if we change this, then you should update the TLA+ model over at
https://git.kernel.org/pub/scm/linux/kernel/git/cmarinas/kernel-tla.git/tree/asidalloc.tla
Cheers,
Will
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prev parent reply other threads:[~2023-09-29 16:51 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-09-27 5:51 [PATCH] arm64: mm: Optimize querying asid from reserved_asids Guo Hui
2023-09-29 16:51 ` Will Deacon [this message]
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