| Message ID | bb4a9de6-3adb-bec0-1bf1-22dd25a8d5c0@huawei.com |
|---|---|
| State | New |
| Headers | show |
| Series | QSBR: Use xor operation to replace add operation when changing rcu_gp.ctr value | expand |
----- On Feb 16, 2022, at 2:35 AM, lttng-dev lttng-dev at lists.lttng.org wrote: > It is enough to have three values of rcu_gp.ctr, 00 for INACTIVE, > 01 or 11 for ACTIVE. So it is possible to replace add operation > with xor operation when changing rcu_gp.ctr value. What is missing here is a description justifying why this change is useful. What is inherently better about XOR compared to ADD or even binary-OR ? If it's about performance, then a benchmark on relevant architectures would be useful. But I suspect that if end users care that much about the performance of urcu_qsbr_synchronize_rcu(), they might be doing something wrong. Thanks, Mathieu > > Signed-off-by: yaowenbin <yaowenbin1 at huawei.com> > --- > src/urcu-qsbr.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/src/urcu-qsbr.c b/src/urcu-qsbr.c > index 3709412..46135f9 100644 > --- a/src/urcu-qsbr.c > +++ b/src/urcu-qsbr.c > @@ -391,7 +391,7 @@ void urcu_qsbr_synchronize_rcu(void) > goto out; > > /* Increment current G.P. */ > - CMM_STORE_SHARED(urcu_qsbr_gp.ctr, urcu_qsbr_gp.ctr + URCU_QSBR_GP_CTR); > + CMM_STORE_SHARED(urcu_qsbr_gp.ctr, urcu_qsbr_gp.ctr ^ URCU_QSBR_GP_CTR); > > /* > * Must commit urcu_qsbr_gp.ctr update to memory before waiting for > -- > 2.27.0 > _______________________________________________ > lttng-dev mailing list > lttng-dev at lists.lttng.org > https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
On Wed, Feb 16, 2022 at 03:53:20PM -0500, Mathieu Desnoyers wrote: > ----- On Feb 16, 2022, at 2:35 AM, lttng-dev lttng-dev at lists.lttng.org wrote: > > > It is enough to have three values of rcu_gp.ctr, 00 for INACTIVE, > > 01 or 11 for ACTIVE. So it is possible to replace add operation > > with xor operation when changing rcu_gp.ctr value. > > What is missing here is a description justifying why this change is useful. > > What is inherently better about XOR compared to ADD or even binary-OR ? > > If it's about performance, then a benchmark on relevant architectures > would be useful. But I suspect that if end users care that much about the > performance of urcu_qsbr_synchronize_rcu(), they might be doing something > wrong. Plus having the full counter can be extremely helpful when debugging. Thanx, Paul > Thanks, > > Mathieu > > > > > Signed-off-by: yaowenbin <yaowenbin1 at huawei.com> > > --- > > src/urcu-qsbr.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/src/urcu-qsbr.c b/src/urcu-qsbr.c > > index 3709412..46135f9 100644 > > --- a/src/urcu-qsbr.c > > +++ b/src/urcu-qsbr.c > > @@ -391,7 +391,7 @@ void urcu_qsbr_synchronize_rcu(void) > > goto out; > > > > /* Increment current G.P. */ > > - CMM_STORE_SHARED(urcu_qsbr_gp.ctr, urcu_qsbr_gp.ctr + URCU_QSBR_GP_CTR); > > + CMM_STORE_SHARED(urcu_qsbr_gp.ctr, urcu_qsbr_gp.ctr ^ URCU_QSBR_GP_CTR); > > > > /* > > * Must commit urcu_qsbr_gp.ctr update to memory before waiting for > > -- > > 2.27.0 > > _______________________________________________ > > lttng-dev mailing list > > lttng-dev at lists.lttng.org > > https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev > > -- > Mathieu Desnoyers > EfficiOS Inc. > http://www.efficios.com
diff --git a/src/urcu-qsbr.c b/src/urcu-qsbr.c index 3709412..46135f9 100644 --- a/src/urcu-qsbr.c +++ b/src/urcu-qsbr.c @@ -391,7 +391,7 @@ void urcu_qsbr_synchronize_rcu(void) goto out; /* Increment current G.P. */ - CMM_STORE_SHARED(urcu_qsbr_gp.ctr, urcu_qsbr_gp.ctr + URCU_QSBR_GP_CTR); + CMM_STORE_SHARED(urcu_qsbr_gp.ctr, urcu_qsbr_gp.ctr ^ URCU_QSBR_GP_CTR); /* * Must commit urcu_qsbr_gp.ctr update to memory before waiting for
It is enough to have three values of rcu_gp.ctr, 00 for INACTIVE, 01 or 11 for ACTIVE. So it is possible to replace add operation with xor operation when changing rcu_gp.ctr value. Signed-off-by: yaowenbin <yaowenbin1 at huawei.com> --- src/urcu-qsbr.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)