| Message ID | 20211015173902.2278118-1-nathanl@linux.ibm.com (mailing list archive) |
|---|---|
| State | Accepted |
| Headers | show |
| Series | [v2] powerpc/smp: do not decrement idle task preempt count in CPU offline | expand |
| Related | show |
| Context | Check | Description |
|---|---|---|
| snowpatch_ozlabs/github-powerpc_selftests | success | Successfully ran 8 jobs. |
| snowpatch_ozlabs/github-powerpc_ppctests | success | Successfully ran 8 jobs. |
| snowpatch_ozlabs/github-powerpc_clang | success | Successfully ran 7 jobs. |
| snowpatch_ozlabs/github-powerpc_kernel_qemu | success | Successfully ran 24 jobs. |
| snowpatch_ozlabs/github-powerpc_sparse | success | Successfully ran 4 jobs. |
* Nathan Lynch <nathanl@linux.ibm.com> [2021-10-15 12:39:02]: > With PREEMPT_COUNT=y, when a CPU is offlined and then onlined again, we > get: > > BUG: scheduling while atomic: swapper/1/0/0x00000000 > no locks held by swapper/1/0. > CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.15.0-rc2+ #100 > Call Trace: > dump_stack_lvl+0xac/0x108 > __schedule_bug+0xac/0xe0 > __schedule+0xcf8/0x10d0 > schedule_idle+0x3c/0x70 > do_idle+0x2d8/0x4a0 > cpu_startup_entry+0x38/0x40 > start_secondary+0x2ec/0x3a0 > start_secondary_prolog+0x10/0x14 > > This is because powerpc's arch_cpu_idle_dead() decrements the idle task's > preempt count, for reasons explained in commit a7c2bb8279d2 ("powerpc: > Re-enable preemption before cpu_die()"), specifically "start_secondary() > expects a preempt_count() of 0." > > However, since commit 2c669ef6979c ("powerpc/preempt: Don't touch the idle > task's preempt_count during hotplug") and commit f1a0a376ca0c ("sched/core: > Initialize the idle task with preemption disabled"), that justification no > longer holds. > > The idle task isn't supposed to re-enable preemption, so remove the > vestigial preempt_enable() from the CPU offline path. > > Tested with pseries and powernv in qemu, and pseries on PowerVM. > > Fixes: 2c669ef6979c ("powerpc/preempt: Don't touch the idle task's preempt_count during hotplug") > Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com> > Reviewed-by: Valentin Schneider <valentin.schneider@arm.com> Looks good to me. Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com> > --- > > Notes: > Changes since v1: > > - remove incorrect Fixes: tag, add Valentin's r-b. > > arch/powerpc/kernel/smp.c | 2 -- > 1 file changed, 2 deletions(-) > > diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c > index 9cc7d3dbf439..605bab448f84 100644 > --- a/arch/powerpc/kernel/smp.c > +++ b/arch/powerpc/kernel/smp.c > @@ -1730,8 +1730,6 @@ void __cpu_die(unsigned int cpu) > > void arch_cpu_idle_dead(void) > { > - sched_preempt_enable_no_resched(); > - > /* > * Disable on the down path. This will be re-enabled by > * start_secondary() via start_secondary_resume() below > -- > 2.31.1 >
On Fri, 15 Oct 2021 12:39:02 -0500, Nathan Lynch wrote: > With PREEMPT_COUNT=y, when a CPU is offlined and then onlined again, we > get: > > BUG: scheduling while atomic: swapper/1/0/0x00000000 > no locks held by swapper/1/0. > CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.15.0-rc2+ #100 > Call Trace: > dump_stack_lvl+0xac/0x108 > __schedule_bug+0xac/0xe0 > __schedule+0xcf8/0x10d0 > schedule_idle+0x3c/0x70 > do_idle+0x2d8/0x4a0 > cpu_startup_entry+0x38/0x40 > start_secondary+0x2ec/0x3a0 > start_secondary_prolog+0x10/0x14 > > [...] Applied to powerpc/fixes. [1/1] powerpc/smp: do not decrement idle task preempt count in CPU offline https://git.kernel.org/powerpc/c/787252a10d9422f3058df9a4821f389e5326c440 cheers
diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c index 9cc7d3dbf439..605bab448f84 100644 --- a/arch/powerpc/kernel/smp.c +++ b/arch/powerpc/kernel/smp.c @@ -1730,8 +1730,6 @@ void __cpu_die(unsigned int cpu) void arch_cpu_idle_dead(void) { - sched_preempt_enable_no_resched(); - /* * Disable on the down path. This will be re-enabled by * start_secondary() via start_secondary_resume() below
With PREEMPT_COUNT=y, when a CPU is offlined and then onlined again, we get: BUG: scheduling while atomic: swapper/1/0/0x00000000 no locks held by swapper/1/0. CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.15.0-rc2+ #100 Call Trace: dump_stack_lvl+0xac/0x108 __schedule_bug+0xac/0xe0 __schedule+0xcf8/0x10d0 schedule_idle+0x3c/0x70 do_idle+0x2d8/0x4a0 cpu_startup_entry+0x38/0x40 start_secondary+0x2ec/0x3a0 start_secondary_prolog+0x10/0x14 This is because powerpc's arch_cpu_idle_dead() decrements the idle task's preempt count, for reasons explained in commit a7c2bb8279d2 ("powerpc: Re-enable preemption before cpu_die()"), specifically "start_secondary() expects a preempt_count() of 0." However, since commit 2c669ef6979c ("powerpc/preempt: Don't touch the idle task's preempt_count during hotplug") and commit f1a0a376ca0c ("sched/core: Initialize the idle task with preemption disabled"), that justification no longer holds. The idle task isn't supposed to re-enable preemption, so remove the vestigial preempt_enable() from the CPU offline path. Tested with pseries and powernv in qemu, and pseries on PowerVM. Fixes: 2c669ef6979c ("powerpc/preempt: Don't touch the idle task's preempt_count during hotplug") Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com> Reviewed-by: Valentin Schneider <valentin.schneider@arm.com> --- Notes: Changes since v1: - remove incorrect Fixes: tag, add Valentin's r-b. arch/powerpc/kernel/smp.c | 2 -- 1 file changed, 2 deletions(-)