From: marc.zyngier@arm•com (Marc Zyngier)
To: linux-arm-kernel@lists•infradead.org
Subject: [PATCH 0/3] New algorithm for ASID allocation and rollover
Date: Wed, 15 Aug 2012 18:05:55 +0100 [thread overview]
Message-ID: <502BD6F3.9080909@arm.com> (raw)
In-Reply-To: <1345049642-29704-1-git-send-email-will.deacon@arm.com>
On 15/08/12 17:53, Will Deacon wrote:
> Hello,
>
> Following some investigation into preempt-rt Linux, it became apparent
> that ASID rollover can happen fairly regularly under certain heavy
> scheduling workloads. Each time this happens, we broadcast an interrupt
> to the secondary CPUs so that we can reset the global ASID numberspace
> without assigning duplicate ASIDs to different tasks or accidentally
> assigning different ASIDs to threads of the same process.
>
> This leads to a large number of expensive IPIs between cores:
>
> CPU0 CPU1
> IPI0: 0 0 Timer broadcast interrupts
> IPI1: 23165 115888 Rescheduling interrupts
> IPI2: 0 0 Function call interrupts
> IPI3: 6619 1123 Single function call interrupts <---- IPIs
> IPI4: 0 0 CPU stop interrupts
>
> Digging deeper, this also leads to an extremely varied waittime on the
> cpu_asid_lock. Granted this is only contended for <1% of the time, but
> the waittime varies between 0.5 and 734 us!
>
> After some discussion, it became apparent that tracking the ASIDs
> currently active on the cores in the system means that, on rollover, we
> can automatically reserve those that are in use without having to stop
> the world.
>
> This patch series develops that idea so that:
>
> - We can support cores without hardware broadcasting of TLB maintenance
> operations without resorting to IPIs.
This particular bit should benefit even more to virtual machines, as it
avoids trapping back to the host to handle a write to the (emulated) GIC
distributor, and the corresponding interrupt injection on the target vcpus.
I'll give it a spin on KVM.
M.
--
Jazz is not dead. It just smells funny...
next prev parent reply other threads:[~2012-08-15 17:05 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-08-15 16:53 [PATCH 0/3] New algorithm for ASID allocation and rollover Will Deacon
2012-08-15 16:54 ` [PATCH 1/3] ARM: mm: remove IPI broadcasting on ASID rollover Will Deacon
2012-08-15 16:54 ` [PATCH 2/3] ARM: mm: avoid taking ASID spinlock on fastpath Will Deacon
2012-08-15 16:54 ` [PATCH 3/3] ARM: mm: use bitmap operations when allocating new ASIDs Will Deacon
2012-08-15 17:05 ` Marc Zyngier [this message]
2012-08-19 15:21 ` [PATCH 0/3] New algorithm for ASID allocation and rollover Arnd Bergmann
2012-08-20 12:51 ` Will Deacon
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