From: Peter Zijlstra <peterz@infradead.org>
To: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
David Woodhouse <dwmw2@infradead.org>,
Paul Durrant <paul@xen.org>, Ingo Molnar <mingo@redhat.com>,
Will Deacon <will@kernel.org>, Boqun Feng <boqun@kernel.org>,
Waiman Long <longman@redhat.com>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
David Woodhouse <dwmw@amazon.co.uk>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
syzbot+208f7f3e5f59c11aeb90@syzkaller.appspotmail.com,
Carsten Stollmaier <stollmc@amazon.com>
Subject: Re: [PATCH v2 01/20] locking/rt: Use raw_spin_lock_irqsave() in __rwbase_read_unlock()
Date: Fri, 29 May 2026 22:38:41 +0200 [thread overview]
Message-ID: <20260529203841.GC3568911@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <20260529201335.GP3493090@noisy.programming.kicks-ass.net>
On Fri, May 29, 2026 at 10:13:35PM +0200, Peter Zijlstra wrote:
> It is somewhat possible to do an RT aware read-write spinlock thing, but
> it is definitely non-trivial and this would be the only user.
Furthermore, it is fundamentally one of the worst possible lock types.
It really isn't something you *want* to have -- arguably even for !RT.
They scale like ass; per them being a spinlock type, the critical
sections must be short, but this means there is nothing to amortize the
cost of bouncing the shared lock around -- which is the 'saving' grace
of the rwsem.
For short sections the cost of the shared access will dominate. I'm sure
Paul has a bunch of graphs to illustrate this point :-)
next prev parent reply other threads:[~2026-05-29 20:39 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-05-29 16:50 [PATCH v2 00/20] KVM: x86/xen: Fix Xen/GP/PREEMPT_RT issues with rwlock_t Sean Christopherson
2026-05-29 16:50 ` [PATCH v2 01/20] locking/rt: Use raw_spin_lock_irqsave() in __rwbase_read_unlock() Sean Christopherson
2026-05-29 19:32 ` Peter Zijlstra
2026-05-29 19:34 ` Peter Zijlstra
2026-05-29 20:05 ` Sean Christopherson
2026-05-29 20:13 ` Peter Zijlstra
2026-05-29 20:38 ` Peter Zijlstra [this message]
2026-05-30 0:54 ` Sean Christopherson
2026-05-30 10:26 ` Paolo Bonzini
2026-05-30 12:47 ` David Woodhouse
2026-05-30 14:40 ` Paolo Bonzini
2026-06-01 10:52 ` David Woodhouse
2026-06-01 13:01 ` David Woodhouse
2026-06-01 13:40 ` Sebastian Andrzej Siewior
2026-06-01 13:53 ` David Woodhouse
2026-06-01 14:47 ` Sebastian Andrzej Siewior
2026-06-01 15:11 ` David Woodhouse
2026-06-01 9:40 ` Peter Zijlstra
2026-06-01 10:04 ` David Woodhouse
2026-05-30 13:02 ` Paolo Bonzini
2026-06-01 8:40 ` Peter Zijlstra
2026-06-01 11:11 ` Sebastian Andrzej Siewior
2026-06-01 11:40 ` Peter Zijlstra
2026-06-01 19:13 ` Paolo Bonzini
2026-06-02 7:34 ` Sebastian Andrzej Siewior
2026-06-04 23:58 ` David Woodhouse
2026-05-29 16:50 ` [PATCH v2 02/20] KVM: x86/xen: Use read_trylock() for GPC locks in hardirq/atomic paths Sean Christopherson
2026-05-29 23:28 ` Hillf Danton
2026-05-29 16:50 ` [PATCH v2 03/20] KVM: x86/xen: Remove unnecessary irqsave from GPC lock usage in xen.c Sean Christopherson
2026-05-29 16:50 ` [PATCH v2 04/20] KVM: x86: Remove unnecessary irqsave from kvm_setup_guest_pvclock() Sean Christopherson
2026-05-29 16:50 ` [PATCH v2 05/20] KVM: Remove unnecessary IRQ disabling from GPC lock in pfncache.c Sean Christopherson
2026-05-29 16:51 ` [PATCH v2 06/20] KVM: x86/xen: Use guard() to grab kvm->srcu around gpc critical sections Sean Christopherson
2026-05-29 16:51 ` [PATCH v2 07/20] KVM: x86/xen: Extract delivery of event to vCPU into a separate helper Sean Christopherson
2026-05-29 16:51 ` [PATCH v2 08/20] KVM: x86/xen: Explicitly tag "shared info" page as never being dirty tracked Sean Christopherson
2026-05-29 16:51 ` [PATCH v2 09/20] KVM: x86/xen: Don't dirty track "vCPU info" page Sean Christopherson
2026-05-29 16:51 ` [PATCH v2 10/20] KVM: Move {g,p}fn <=> {g,h}pa conversion helpers to kvm_types.h Sean Christopherson
2026-05-29 16:51 ` [PATCH v2 11/20] KVM: Add CLASS() constructs to automagically handle lock+check of gpc Sean Christopherson
2026-05-29 16:51 ` [PATCH v2 12/20] KVM: x86/xen: Convert kvm_xen_shared_info_init() to gpc's CLASS() APIs Sean Christopherson
2026-05-29 16:51 ` [PATCH v2 13/20] KVM: x86/xen: Don't bother waiting on gpc->lock in SCHEDOP_poll Sean Christopherson
2026-05-29 16:51 ` [PATCH v2 14/20] KVM: x86/xen: Convert wait_pending_event() to gpc's CLASS() APIs Sean Christopherson
2026-05-29 16:51 ` [PATCH v2 15/20] KVM: x86/xen: Convert kvm_xen_set_evtchn_fast() " Sean Christopherson
2026-05-29 16:51 ` [PATCH v2 16/20] KVM: x86/xen: Convert xen_get_guest_pvclock() " Sean Christopherson
2026-05-29 16:51 ` [PATCH v2 17/20] KVM: x86/xen: Drop local "kick_vcpu" from __kvm_xen_set_evtchn_fast() Sean Christopherson
2026-05-29 16:51 ` [PATCH v2 18/20] KVM: x86/xen: Convert event injection to gpc's CLASS() APIs Sean Christopherson
2026-05-29 16:51 ` [PATCH v2 19/20] KVM: Add "extended" gpc CLASS() APIs for sometimes-atomic cases Sean Christopherson
2026-05-29 16:51 ` [PATCH v2 20/20] KVM: x86: Use gfn_to_pfn_cache for record_steal_time Sean Christopherson
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