From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754238AbZBIDrx (ORCPT ); Sun, 8 Feb 2009 22:47:53 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753843AbZBIDrp (ORCPT ); Sun, 8 Feb 2009 22:47:45 -0500 Received: from e8.ny.us.ibm.com ([32.97.182.138]:33323 "EHLO e8.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753833AbZBIDro (ORCPT ); Sun, 8 Feb 2009 22:47:44 -0500 Date: Sun, 8 Feb 2009 19:47:48 -0800 From: "Paul E. McKenney" To: Mathieu Desnoyers Cc: ltt-dev@lists.casi.polymtl.ca, linux-kernel@vger.kernel.org Subject: Re: [ltt-dev] [RFC git tree] Userspace RCU (urcu) for Linux (repost) Message-ID: <20090209034748.GP7120@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20090206130640.GB10918@linux.vnet.ibm.com> <20090206163432.GF10918@linux.vnet.ibm.com> <20090207151028.GA11150@linux.vnet.ibm.com> <20090207233827.GA3557@Krystal> <20090208004416.GH7120@linux.vnet.ibm.com> <20090208214610.GB17569@Krystal> <20090208223606.GM7120@linux.vnet.ibm.com> <20090209002426.GA20025@linux.vnet.ibm.com> <20090209005450.GB22089@Krystal> <20090209010825.GA23462@Krystal> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090209010825.GA23462@Krystal> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Feb 08, 2009 at 08:08:25PM -0500, Mathieu Desnoyers wrote: > * Mathieu Desnoyers (compudj@krystal.dyndns.org) wrote: > > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > > > On Sun, Feb 08, 2009 at 02:36:06PM -0800, Paul E. McKenney wrote: > > > > On Sun, Feb 08, 2009 at 04:46:10PM -0500, Mathieu Desnoyers wrote: > > > > > > [ . . . ] > > > > > > > > I ran your modified version within my benchmarks : > > > > > > > > > > with return value : 14.164 cycles per read > > > > > without return value : 16.4017 cycles per read > > > > > > > > > > So we have a 14% performance decrease due to this. We also pollute the > > > > > branch prediction buffer and we add a cache access due to the added > > > > > variables in the TLS. Returning the value has the clear advantage of > > > > > letting the compiler keep it around in registers or on the stack, which > > > > > clearly costs less. > > > > > > > > > > So I think the speed factor outweights the visual considerations. Maybe > > > > > we could switch to something like : > > > > > > > > > > unsigned int qparity; > > > > > > > > > > urcu_read_lock(&qparity); > > > > > ... > > > > > urcu_read_unlock(&qparity); > > > > > > > > > > That would be a bit like local_irq_save() in the kernel, except that we > > > > > could do it in a static inline because we pass the address. I > > > > > personnally dislike the local_irq_save() way of hiding the fact that it > > > > > writes to the variable in a "clever" macro. I'd really prefer to leave > > > > > the " & ". > > > > > > > > > > What is your opinion ? > > > > > > > > My current opinion is that I can avoid the overflow problem and the > > > > need to recheck, which might get rid of the need for both arguments > > > > and return values while still maintaining good performance. The trick > > > > is to use only the topmost bit for the grace-period counter, and all > > > > the rest of the bits for nesting. That way, no matter what value of > > > > global counter one picks up, it will be waited for (since there are but > > > > two values that the global counter takes on). > > > > > > > > But just now coding it, so will see if it actually works. > > > > > > Seems to work, and seems to be pretty fast on my machine, anyway. > > > This one adapts itself to 32- and 64-bit machines, though almost > > > all of the code is common. It does do a check, but avoids array > > > indexing, arguments, and return values. > > > > > > How does it do on your hardware? > > > > > > Signed-off-by: Paul E. McKenney > > > > Wow... > > > > Patch updated against HEAD. > > > > Time per read : 7.53622 cycles > > > > Half of what we had previously.. I'll have to look at the assembly. :) > > > > Signed-off-by: Paul E. McKenney > > Signed-off-by: Mathieu Desnoyers > > --- > > > > test_urcu.c | 6 +++--- > > test_urcu_timing.c | 6 +++--- > > urcu.c | 23 ++++++++++------------- > > urcu.h | 42 +++++++++++++++++++++++++++++------------- > > 4 files changed, 45 insertions(+), 32 deletions(-) > > > > diff --git a/test_urcu.c b/test_urcu.c > > index f6be45b..f115a4a 100644 > > --- a/test_urcu.c > > +++ b/test_urcu.c > > @@ -72,7 +72,7 @@ void rcu_copy_mutex_unlock(void) > > > > void *thr_reader(void *arg) > > { > > - int qparity, i, j; > > + int i, j; > > struct test_array *local_ptr; > > > > printf("thread %s, thread id : %lx, tid %lu\n", > > @@ -83,14 +83,14 @@ void *thr_reader(void *arg) > > > > for (i = 0; i < 100000; i++) { > > for (j = 0; j < 100000000; j++) { > > - rcu_read_lock(&qparity); > > + rcu_read_lock(); > > local_ptr = rcu_dereference(test_rcu_pointer); > > if (local_ptr) { > > assert(local_ptr->a == 8); > > assert(local_ptr->b == 12); > > assert(local_ptr->c[55] == 2); > > } > > - rcu_read_unlock(&qparity); > > + rcu_read_unlock(); > > } > > } > > > > diff --git a/test_urcu_timing.c b/test_urcu_timing.c > > index 57fda4f..9903705 100644 > > --- a/test_urcu_timing.c > > +++ b/test_urcu_timing.c > > @@ -94,7 +94,7 @@ static cycles_t reader_time[NR_READ] __attribute__((aligned(128))); > > > > void *thr_reader(void *arg) > > { > > - int qparity, i, j; > > + int i, j; > > struct test_array *local_ptr; > > cycles_t time1, time2; > > > > @@ -107,12 +107,12 @@ void *thr_reader(void *arg) > > time1 = get_cycles(); > > for (i = 0; i < OUTER_READ_LOOP; i++) { > > for (j = 0; j < INNER_READ_LOOP; j++) { > > - rcu_read_lock(&qparity); > > + rcu_read_lock(); > > local_ptr = rcu_dereference(test_rcu_pointer); > > if (local_ptr) { > > assert(local_ptr->a == 8); > > } > > - rcu_read_unlock(&qparity); > > + rcu_read_unlock(); > > } > > } > > time2 = get_cycles(); > > diff --git a/urcu.c b/urcu.c > > index 08fb75d..2914b66 100644 > > --- a/urcu.c > > +++ b/urcu.c > > @@ -19,17 +19,17 @@ > > > > pthread_mutex_t urcu_mutex = PTHREAD_MUTEX_INITIALIZER; > > > > -/* Global quiescent period parity */ > > -int urcu_qparity; > > +/* Global grace period counter */ > > +long urcu_gp_ctr; > > > > -int __thread urcu_active_readers[2]; > > +long __thread urcu_active_readers; > > > > /* Thread IDs of registered readers */ > > #define INIT_NUM_THREADS 4 > > > > struct reader_data { > > pthread_t tid; > > - int *urcu_active_readers; > > + long *urcu_active_readers; > > }; > > > > static struct reader_data *reader_data; > > @@ -60,11 +60,9 @@ void internal_urcu_unlock(void) > > /* > > * called with urcu_mutex held. > > */ > > -static int switch_next_urcu_qparity(void) > > +static void switch_next_urcu_qparity(void) > > { > > - int old_parity = urcu_qparity; > > - urcu_qparity = 1 - old_parity; > > - return old_parity; > > + urcu_gp_ctr += RCU_GP_CTR_BOTTOM_BIT; > > } > > > > static void force_mb_all_threads(void) > > @@ -89,7 +87,7 @@ static void force_mb_all_threads(void) > > mb(); /* read sig_done before ending the barrier */ > > } > > > > -void wait_for_quiescent_state(int parity) > > +void wait_for_quiescent_state(void) > > { > > struct reader_data *index; > > > > @@ -101,7 +99,7 @@ void wait_for_quiescent_state(int parity) > > /* > > * BUSY-LOOP. > > */ > > - while (index->urcu_active_readers[parity] != 0) > > + while (rcu_old_gp_ongoing(index->urcu_active_readers)) > > barrier(); > > } > > /* > > @@ -115,17 +113,16 @@ void wait_for_quiescent_state(int parity) > > > > static void switch_qparity(void) > > { > > - int prev_parity; > > > > /* All threads should read qparity before accessing data structure. */ > > /* Write ptr before changing the qparity */ > > force_mb_all_threads(); > > - prev_parity = switch_next_urcu_qparity(); > > + switch_next_urcu_qparity(); > > > > /* > > * Wait for previous parity to be empty of readers. > > */ > > - wait_for_quiescent_state(prev_parity); > > + wait_for_quiescent_state(); > > } > > > > void synchronize_rcu(void) > > diff --git a/urcu.h b/urcu.h > > index b6b5c7b..e83c69f 100644 > > --- a/urcu.h > > +++ b/urcu.h > > @@ -66,23 +66,39 @@ static inline void atomic_inc(int *v) > > > > #define SIGURCU SIGUSR1 > > > > -/* Global quiescent period parity */ > > -extern int urcu_qparity; > > +#define RCU_GP_CTR_BOTTOM_BIT (sizeof(long) == 4 ? 0x80000000 : 0x100L) > > Shouldn't it be the opposite ? > > e.g. > > #define RCU_GP_CTR_BOTTOM_BIT (sizeof(long) == 4 ? 0x100L : 0x80000000L) Absolutely not!!! For 32-bit systems, the GP count is only the upper bit. That is exactly what allows the overflow check to be omitted. For 64-bit systems, I rely on the upper 56 bits taking a couple of millenia to overflow. For 64-bit systems, one could also use only the upper bit (0x8000000000000000), and that might actually make for better code. > > +#define RCU_GP_CTR_NEST_MASK (RCU_GP_CTR_BOTTOM_BIT - 1) > > > > -extern int __thread urcu_active_readers[2]; > > +/* Global quiescent period counter with low-order bits unused. */ > > +extern long urcu_gp_ctr; > > > > -static inline int get_urcu_qparity(void) > > +extern long __thread urcu_active_readers; > > + > > +static inline int rcu_old_gp_ongoing(long *value) > > { > > - return urcu_qparity; > > + long v; > > + > > + if (value == NULL) > > + return 0; > > + v = ACCESS_ONCE(*value); > > + if (sizeof(long) == 4) { > > + return (v & RCU_GP_CTR_NEST_MASK) && > > + ((v ^ ACCESS_ONCE(urcu_gp_ctr)) & ~RCU_GP_CTR_NEST_MASK); > > There must be something about the ^ I am missing ? Compared to it, the > 64-bits test is a - , with < 0... Yep. For 32 bits, if the top bit is the same as that of the current value of the counter, we must wait. I could have written: (v & ~RCU_GP_CTR_NEST_MASK) != (ACCESS_ONCE(urcu_gp_ctr) & ~RCU_GP_CTR_NEST_MASK) but doing so would require two "&" operations. Though perhaps the compiler would have figured it out... Thanx, Paul > Mathieu > > > + } else { > > + return (v & RCU_GP_CTR_NEST_MASK) && > > + (v - ACCESS_ONCE(urcu_gp_ctr) < 0); > > + } > > } > > > > -/* > > - * urcu_parity should be declared on the caller's stack. > > - */ > > -static inline void rcu_read_lock(int *urcu_parity) > > +static inline void rcu_read_lock(void) > > { > > - *urcu_parity = get_urcu_qparity(); > > - urcu_active_readers[*urcu_parity]++; > > + long tmp; > > + > > + tmp = urcu_active_readers; > > + if ((tmp & RCU_GP_CTR_NEST_MASK) == 0) > > + urcu_active_readers = urcu_gp_ctr + 1; > > + else > > + urcu_active_readers = tmp + 1; > > /* > > * Increment active readers count before accessing the pointer. > > * See force_mb_all_threads(). > > @@ -90,14 +106,14 @@ static inline void rcu_read_lock(int *urcu_parity) > > barrier(); > > } > > > > -static inline void rcu_read_unlock(int *urcu_parity) > > +static inline void rcu_read_unlock(void) > > { > > barrier(); > > /* > > * Finish using rcu before decrementing the pointer. > > * See force_mb_all_threads(). > > */ > > - urcu_active_readers[*urcu_parity]--; > > + urcu_active_readers--; > > } > > > > extern void *urcu_publish_content(void **ptr, void *new); > > > > > > > > -- > > Mathieu Desnoyers > > OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 > > > > _______________________________________________ > > ltt-dev mailing list > > ltt-dev@lists.casi.polymtl.ca > > http://lists.casi.polymtl.ca/cgi-bin/mailman/listinfo/ltt-dev > > > > -- > Mathieu Desnoyers > OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68