From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <waiman.long@hp.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
linux-arch@vger.kernel.org, x86@kernel.org,
linux-kernel@vger.kernel.org,
virtualization@lists.linux-foundation.org,
xen-devel@lists.xenproject.org, kvm@vger.kernel.org,
Paolo Bonzini <paolo.bonzini@gmail.com>,
Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
Boris Ostrovsky <boris.ostrovsky@oracle.com>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Rik van Riel <riel@redhat.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>,
David Vrabel <david.vrabel@citrix.com>,
Oleg Nesterov <oleg@redhat.com>,
Daniel J Blueman <daniel@numascale.com>,
Scott J Norton <scott.norton@hp.com>,
Douglas Hatch <doug.hatch@hp.com>
Subject: Re: [PATCH v15 09/15] pvqspinlock: Implement simple paravirt support for the qspinlock
Date: Mon, 13 Apr 2015 16:47:29 +0200 [thread overview]
Message-ID: <20150413144729.GG5029@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <5526F218.2070909@hp.com>
On Thu, Apr 09, 2015 at 05:41:44PM -0400, Waiman Long wrote:
> >>+void __init __pv_init_lock_hash(void)
> >>+{
> >>+ int pv_hash_size = 4 * num_possible_cpus();
> >>+
> >>+ if (pv_hash_size< (1U<< LFSR_MIN_BITS))
> >>+ pv_hash_size = (1U<< LFSR_MIN_BITS);
> >>+ /*
> >>+ * Allocate space from bootmem which should be page-size aligned
> >>+ * and hence cacheline aligned.
> >>+ */
> >>+ pv_lock_hash = alloc_large_system_hash("PV qspinlock",
> >>+ sizeof(struct pv_hash_bucket),
> >>+ pv_hash_size, 0, HASH_EARLY,
> >>+ &pv_lock_hash_bits, NULL,
> >>+ pv_hash_size, pv_hash_size);
> > pv_taps = lfsr_taps(pv_lock_hash_bits);
> >
>
> I don't understand what you meant here.
Let me explain (even though I propose taking all the LFSR stuff out).
pv_lock_hash_bit is a runtime variable, therefore it cannot compile time
evaluate the forest of if statements required to compute the taps value.
Therefore its best to compute the taps _once_, and this seems like a
good place to do so.
> >>+ goto done;
> >>+ }
> >>+ }
> >>+
> >>+ hash = lfsr(hash, pv_lock_hash_bits, 0);
> >Since pv_lock_hash_bits is a variable, you end up running through that
> >massive if() forest to find the corresponding tap every single time. It
> >cannot compile-time optimize it.
>
> The minimum bits size is now 8. So unless the system has more than 64 vCPUs,
> it will get the right value in the first if statement.
Still, no reason to not pre-compute the taps value, its simple enough.
next prev parent reply other threads:[~2015-04-13 14:47 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-04-07 2:55 [PATCH v15 00/15] qspinlock: a 4-byte queue spinlock with PV support Waiman Long
2015-04-07 2:55 ` [PATCH v15 01/15] qspinlock: A simple generic 4-byte queue spinlock Waiman Long
2015-04-07 2:55 ` [PATCH v15 02/15] qspinlock, x86: Enable x86-64 to use " Waiman Long
2015-04-07 2:55 ` [PATCH v15 03/15] qspinlock: Add pending bit Waiman Long
2015-04-07 2:55 ` [PATCH v15 04/15] qspinlock: Extract out code snippets for the next patch Waiman Long
2015-04-07 2:55 ` [PATCH v15 05/15] qspinlock: Optimize for smaller NR_CPUS Waiman Long
2015-04-07 2:55 ` [PATCH v15 06/15] qspinlock: Use a simple write to grab the lock Waiman Long
2015-04-07 2:55 ` [PATCH v15 07/15] qspinlock: Revert to test-and-set on hypervisors Waiman Long
2015-04-07 2:55 ` [PATCH v15 08/15] lfsr: a simple binary Galois linear feedback shift register Waiman Long
2015-04-07 2:55 ` [PATCH v15 09/15] pvqspinlock: Implement simple paravirt support for the qspinlock Waiman Long
2015-04-09 18:13 ` Peter Zijlstra
2015-04-09 18:23 ` Peter Zijlstra
2015-04-09 20:36 ` Waiman Long
2015-04-09 21:41 ` Waiman Long
2015-04-13 14:47 ` Peter Zijlstra [this message]
2015-04-13 15:45 ` Waiman Long
2015-04-13 15:08 ` Peter Zijlstra
2015-04-13 15:51 ` Waiman Long
2015-04-13 15:09 ` Peter Zijlstra
2015-04-13 16:19 ` Waiman Long
2015-04-07 2:55 ` [PATCH v15 10/15] pvqspinlock: Implement the paravirt qspinlock for x86 Waiman Long
2015-04-07 2:55 ` [PATCH v15 11/15] pvqspinlock, x86: Enable PV qspinlock for KVM Waiman Long
2015-04-07 2:55 ` [PATCH v15 12/15] pvqspinlock, x86: Enable PV qspinlock for Xen Waiman Long
2015-04-08 12:01 ` [Xen-devel] " David Vrabel
2015-04-08 17:42 ` Waiman Long
2015-04-07 2:55 ` [PATCH v15 13/15] pvqspinlock: Only kick CPU at unlock time Waiman Long
2015-04-09 19:57 ` Peter Zijlstra
2015-04-09 20:07 ` Peter Zijlstra
2015-04-09 22:06 ` Waiman Long
2015-04-07 2:55 ` [PATCH v15 14/15] pvqspinlock: Improve slowpath performance by avoiding cmpxchg Waiman Long
2015-04-07 2:55 ` [PATCH v15 15/15] pvqspinlock: Add debug code to check for PV lock hash sanity Waiman Long
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