From: Robin Murphy <robin.murphy@arm.com>
To: Will Deacon <will@kernel.org>
Cc: Russell King - ARM Linux admin <linux@armlinux.org.uk>,
linux-arm-kernel@lists.infradead.org
Subject: Re: SMMU performance
Date: Wed, 2 Oct 2019 12:09:13 +0100 [thread overview]
Message-ID: <7e85cfeb-138e-2651-800f-c878b707e2d9@arm.com> (raw)
In-Reply-To: <20191002090233.rqpmhyax2qztyhsr@willie-the-truck>
On 02/10/2019 10:02, Will Deacon wrote:
> On Mon, Sep 30, 2019 at 01:00:00PM +0100, Robin Murphy wrote:
>> On 30/09/2019 12:54, Will Deacon wrote:
>>> On Mon, Sep 30, 2019 at 12:45:28PM +0100, Robin Murphy wrote:
>>>> The major changes in 5.4 are for SMMUv3, so won't impact your platform.
>>>
>>> I was wondering whether rigging up the gather stuff would help here but,
>>> looking at the backtrace, the time is spent on the sync itself so I suspect
>>> it won't help. Hmm... I wonder if we can do better using a sequence number
>>> so that we can ride off the back of somebody else's sync?
>>
>> The trouble with v2 is that then we'd have to introduce locking around the
>> invalidates as well, in order to keep track of what the last 'command'
>> issued in each context was - that's almost certainly going to have far more
>> overhead than eliding syncs could possibly save.
>
> I was thinking along the lines of allocating an ID to each flush, and then
> updating a sync ID on sync, so you can elide the sync if the sync ID is
> greater than your flush ID. But it's vague and I didn't try to implement
> anything.
I don't think that works:
A B
start flush 1
TLBI
start flush 2
TLBI
TLBI
SYNC(2)
TLBI
TLBI
...
SYNC(1)
Even considering your idea upside-down, it seems unlikely to be
beneficial for thread B to sit and wait however long for sync 1 to be
issued just to nominally save issuing sync 2.
Robin.
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prev parent reply other threads:[~2019-10-02 11:09 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-30 11:00 SMMU performance Russell King - ARM Linux admin
2019-09-30 11:45 ` Robin Murphy
2019-09-30 11:54 ` Will Deacon
2019-09-30 12:00 ` Robin Murphy
2019-10-02 9:02 ` Will Deacon
2019-10-02 11:09 ` Robin Murphy [this message]
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