search for: cpu_relax

...d1c0eb51ef87ff2eab5c994949e69: Linux 4.9-rc2 (2016-10-23 17:10:14 -0700) are available in the git repository at: git://git.kernel.org/pub/scm/linux/kernel/git/borntraeger/linux.git tags/cpurelax for you to fetch changes up to dcc37f9044436438360402714b7544a8e8779b07: processor.h: remove cpu_relax_lowlatency (2016-10-25 09:49:57 +0200) ---------------------------------------------------------------- cpu_relax: drop lowlatency, introduce yield For spinning loops people do often use barrier() or cpu_relax(). For most architectures cpu_relax and barrier are the same, but on some architectures...

...d1c0eb51ef87ff2eab5c994949e69: Linux 4.9-rc2 (2016-10-23 17:10:14 -0700) are available in the git repository at: git://git.kernel.org/pub/scm/linux/kernel/git/borntraeger/linux.git tags/cpurelax for you to fetch changes up to dcc37f9044436438360402714b7544a8e8779b07: processor.h: remove cpu_relax_lowlatency (2016-10-25 09:49:57 +0200) ---------------------------------------------------------------- cpu_relax: drop lowlatency, introduce yield For spinning loops people do often use barrier() or cpu_relax(). For most architectures cpu_relax and barrier are the same, but on some architectures...

For spinning loops people do often use barrier() or cpu_relax(). For most architectures cpu_relax and barrier are the same, but on some architectures cpu_relax can add some latency. For example on power,sparc64 and arc, cpu_relax can shift the CPU towards other hardware threads in an SMT environment. On s390 cpu_relax does even more, it uses an hypercall to t...

As there are no users left, we can remove cpu_relax_lowlatency. Signed-off-by: Christian Borntraeger <borntraeger at de.ibm.com> --- arch/alpha/include/asm/processor.h | 1 - arch/arc/include/asm/processor.h | 2 -- arch/arm/include/asm/processor.h | 1 - arch/arm64/include/asm/processor.h | 1 - arch/avr32/include/as...

No need to duplicate the same define everywhere. Since the only user is stop-machine and the only provider is s390, we can use a default implementation of cpu_relax_yield in sched.h. Suggested-by: Russell King <linux at armlinux.org.uk> Signed-off-by: Christian Borntraeger <borntraeger at de.ibm.com> --- arch/alpha/include/asm/processor.h | 1 - arch/arc/include/asm/processor.h | 3 --- arch/arm/include/asm/processor.h | 2 --...

No need to duplicate the same define everywhere. Since the only user is stop-machine and the only provider is s390, we can use a default implementation of cpu_relax_yield in sched.h. Suggested-by: Russell King <linux at armlinux.org.uk> Signed-off-by: Christian Borntraeger <borntraeger at de.ibm.com> --- arch/alpha/include/asm/processor.h | 1 - arch/arc/include/asm/processor.h | 3 --- arch/arm/include/asm/processor.h | 2 --...

On 10/21/2016 04:57 PM, David Miller wrote: > From: Christian Borntraeger <borntraeger at de.ibm.com> > Date: Fri, 21 Oct 2016 13:58:53 +0200 > >> For spinning loops people did often use barrier() or cpu_relax(). >> For most architectures cpu_relax and barrier are the same, but on >> some architectures cpu_relax can add some latency. For example on s390 >> cpu_relax gives up the time slice to the hypervisor. On power cpu_relax >> tries to give some of the CPU to the neighbor threa...

On 10/21/2016 04:57 PM, David Miller wrote: > From: Christian Borntraeger <borntraeger at de.ibm.com> > Date: Fri, 21 Oct 2016 13:58:53 +0200 > >> For spinning loops people did often use barrier() or cpu_relax(). >> For most architectures cpu_relax and barrier are the same, but on >> some architectures cpu_relax can add some latency. For example on s390 >> cpu_relax gives up the time slice to the hypervisor. On power cpu_relax >> tries to give some of the CPU to the neighbor threa...

On Tue, Oct 25, 2016 at 11:03:11AM +0200, Christian Borntraeger wrote: > For spinning loops people do often use barrier() or cpu_relax(). > For most architectures cpu_relax and barrier are the same, but on > some architectures cpu_relax can add some latency. > For example on power,sparc64 and arc, cpu_relax can shift the CPU > towards other hardware threads in an SMT environment. > On s390 cpu_relax does even more,...

On Tue, Oct 25, 2016 at 11:03:11AM +0200, Christian Borntraeger wrote: > For spinning loops people do often use barrier() or cpu_relax(). > For most architectures cpu_relax and barrier are the same, but on > some architectures cpu_relax can add some latency. > For example on power,sparc64 and arc, cpu_relax can shift the CPU > towards other hardware threads in an SMT environment. > On s390 cpu_relax does even more,...

With the s390 special case of a yielding cpu_relax implementation gone, we can now remove all users of cpu_relax_lowlatency and replace them with cpu_relax. Signed-off-by: Christian Borntraeger <borntraeger at de.ibm.com> --- drivers/gpu/drm/i915/i915_gem_request.c | 2 +- drivers/vhost/net.c | 4 ++-- kernel/locking/mcs...

On Tue, Nov 15, 2016 at 02:19:53PM +0100, Christian Borntraeger wrote: > On 11/15/2016 01:30 PM, Russell King - ARM Linux wrote: > > On Tue, Oct 25, 2016 at 11:03:11AM +0200, Christian Borntraeger wrote: > >> For spinning loops people do often use barrier() or cpu_relax(). > >> For most architectures cpu_relax and barrier are the same, but on > >> some architectures cpu_relax can add some latency. > >> For example on power,sparc64 and arc, cpu_relax can shift the CPU > >> towards other hardware threads in an SMT environment. &gt...

On Tue, Nov 15, 2016 at 02:19:53PM +0100, Christian Borntraeger wrote: > On 11/15/2016 01:30 PM, Russell King - ARM Linux wrote: > > On Tue, Oct 25, 2016 at 11:03:11AM +0200, Christian Borntraeger wrote: > >> For spinning loops people do often use barrier() or cpu_relax(). > >> For most architectures cpu_relax and barrier are the same, but on > >> some architectures cpu_relax can add some latency. > >> For example on power,sparc64 and arc, cpu_relax can shift the CPU > >> towards other hardware threads in an SMT environment. &gt...

stop_machine seemed to be the only important place for yielding during cpu_relax. This was fixed by using cpu_relax_yield. Therefore, we can now redefine cpu_relax to be a barrier instead on s390, making s390 identical to all other architectures. Signed-off-by: Christian Borntraeger <borntraeger at de.ibm.com> --- arch/s390/include/asm/processor.h | 2 +- 1 file changed...

...============ --- head-2007-11-30.orig/arch/i386/kernel/smp-xen.c 2007-12-06 14:56:17.000000000 +0100 +++ head-2007-11-30/arch/i386/kernel/smp-xen.c 2007-12-04 09:36:41.000000000 +0100 @@ -525,11 +525,11 @@ /* Wait for response */ while (atomic_read(&data.started) != cpus) - barrier(); + cpu_relax(); if (wait) while (atomic_read(&data.finished) != cpus) - barrier(); + cpu_relax(); spin_unlock(&call_lock); return 0; Index: head-2007-11-30/arch/x86_64/kernel/smp-xen.c =================================================================== --- head-2007-11-30.orig/arch/x86_...

...AL|_Q_PENDING_VAL)) { > + /* > + * If both the lock and pending bits are set, we wait > + * a while to see if that either bit will be cleared. > + * If that is no change, we return and be queued. > + */ > + if (!retry) > + return 0; > + retry--; > + cpu_relax(); > + cpu_relax(); > + *pval = val = atomic_read(&lock->val); > + continue; > + } else if (val == _Q_PENDING_VAL) { > + /* > + * Pending bit is set, but not the lock bit. > + * Assuming that the pending bit holder is going to > + * set the lock bit...

...AL|_Q_PENDING_VAL)) { > + /* > + * If both the lock and pending bits are set, we wait > + * a while to see if that either bit will be cleared. > + * If that is no change, we return and be queued. > + */ > + if (!retry) > + return 0; > + retry--; > + cpu_relax(); > + cpu_relax(); > + *pval = val = atomic_read(&lock->val); > + continue; > + } else if (val == _Q_PENDING_VAL) { > + /* > + * Pending bit is set, but not the lock bit. > + * Assuming that the pending bit holder is going to > + * set the lock bit...

On 11/15/2016 01:30 PM, Russell King - ARM Linux wrote: > On Tue, Oct 25, 2016 at 11:03:11AM +0200, Christian Borntraeger wrote: >> For spinning loops people do often use barrier() or cpu_relax(). >> For most architectures cpu_relax and barrier are the same, but on >> some architectures cpu_relax can add some latency. >> For example on power,sparc64 and arc, cpu_relax can shift the CPU >> towards other hardware threads in an SMT environment. >> On s390 cpu_re...

...rote: > On Tue, Nov 15, 2016 at 02:19:53PM +0100, Christian Borntraeger wrote: >> On 11/15/2016 01:30 PM, Russell King - ARM Linux wrote: >>> On Tue, Oct 25, 2016 at 11:03:11AM +0200, Christian Borntraeger wrote: >>>> For spinning loops people do often use barrier() or cpu_relax(). >>>> For most architectures cpu_relax and barrier are the same, but on >>>> some architectures cpu_relax can add some latency. >>>> For example on power,sparc64 and arc, cpu_relax can shift the CPU >>>> towards other hardware threads in an SMT envi...

...ometimes the hang resolved itself after some minutes) Changing stop_machine to yield the cpu to the hypervisor when yielding inside the guest fixed the problem for me. While I am not completely happy with this patch, I think it causes no harm and it really improves the situation for me. I used cpu_relax for yielding to the hypervisor, does that work on all architectures? p.s.: If you want to reproduce the problem, cpu hotplug and kprobes use stop_machine_run and both triggered the problem after some retries. Signed-off-by: Christian Borntraeger <borntraeger at de.ibm.com> CC: Ingo Moln...