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From: "Radim Krčmář" <rkrcmar@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	Gleb Natapov <gleb@kernel.org>,
	Raghavendra KT <raghavendra.kt@linux.vnet.ibm.com>,
	Vinod Chegu <chegu_vinod@hp.com>, Hui-Zhi <hui-zhi.zhao@hp.com>
Subject: Re: [PATCH 9/9] KVM: VMX: automatic PLE window maximum
Date: Wed, 20 Aug 2014 18:01:38 +0200	[thread overview]
Message-ID: <20140820160137.GB20453@potion.brq.redhat.com> (raw)
In-Reply-To: <53F4BFF3.3060606@redhat.com>

2014-08-20 17:34+0200, Paolo Bonzini:
> Il 20/08/2014 17:31, Radim Krčmář ha scritto:
> > Btw. without extra code, we are still going to overflow on races when
> > changing PW_grow, should they be covered as well?
> 
> You mean because there is no spinlock or similar protecting the changes?
>  I guess you could use a seqlock.

Yes, for example between a modification of ple_window
  new = min(old, PW_actual_max) * PW_grow
which gets compiled into something like this:
  1) tmp = min(old, PW_actual_max)
  2) new = tmp * PW_grow
and a write to increase PW_grow
  3) PW_actual_max = min(PW_max / new_PW_grow, PW_actual_max)
  4) PW_grow = new_PW_grow
  5) PW_actual_max = PW_max / new_PW_grow

3 and 4 can exectute between 1 and 2, which could overflow.

I don't think they are important enough to warrant a significant
performance hit of locking.
Or even more checks that would prevent it in a lockless way.

(I'd just see that the result is set to something legal and also drop
 line 3, because it does not help things that much.)

  reply	other threads:[~2014-08-20 16:01 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-08-19 20:35 [PATCH 0/9] Dynamic Pause Loop Exiting window Radim Krčmář
2014-08-19 20:35 ` [PATCH 1/9] KVM: add kvm_arch_sched_in Radim Krčmář
2014-08-20  7:47   ` Christian Borntraeger
2014-08-20 12:56     ` Radim Krčmář
2014-08-19 20:35 ` [PATCH 2/9] KVM: x86: introduce sched_in to kvm_x86_ops Radim Krčmář
2014-08-19 20:35 ` [PATCH 3/9] KVM: VMX: make PLE window per-vcpu Radim Krčmář
2014-08-20  7:13   ` Paolo Bonzini
2014-08-20 12:26     ` Radim Krčmář
2014-08-19 20:35 ` [PATCH 4/9] KVM: VMX: dynamise PLE window Radim Krčmář
2014-08-19 20:35 ` [PATCH 5/9] KVM: VMX: clamp " Radim Krčmář
2014-08-20  7:18   ` Paolo Bonzini
2014-08-20 12:46     ` Radim Krčmář
2014-08-19 20:35 ` [PATCH 6/9] KVM: trace kvm_ple_window grow/shrink Radim Krčmář
2014-08-19 20:35 ` [PATCH 7/9] KVM: VMX: abstract ple_window modifiers Radim Krčmář
2014-08-20  7:02   ` Paolo Bonzini
2014-08-20 12:25     ` Radim Krčmář
2014-08-19 20:35 ` [PATCH 8/9] KVM: VMX: runtime knobs for dynamic PLE window Radim Krčmář
2014-08-19 20:35 ` [PATCH 9/9] KVM: VMX: automatic PLE window maximum Radim Krčmář
2014-08-20  7:16   ` Paolo Bonzini
2014-08-20  7:18     ` Paolo Bonzini
2014-08-20 12:41     ` Radim Krčmář
2014-08-20 13:15       ` Paolo Bonzini
2014-08-20 15:31         ` Radim Krčmář
2014-08-20 15:34           ` Paolo Bonzini
2014-08-20 16:01             ` Radim Krčmář [this message]
2014-08-20 16:03               ` Paolo Bonzini
2014-08-20 16:26                 ` Radim Krčmář
2014-08-21  6:48 ` [PATCH 0/9] Dynamic Pause Loop Exiting window Zhao, Hui-Zhi (Steven, HPservers-Core-OE-PSC)
2014-08-21  6:48   ` Zhao, Hui-Zhi (Steven, HPservers-Core-OE-PSC)

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