Re: [PATCH 0/9] perf: Avoid explicit cpumask var allocation from stack
From: Dawei Li
Date: Wed Apr 03 2024 - 06:43:57 EST
On Tue, Apr 02, 2024 at 03:41:51PM +0100, Mark Rutland wrote:
> On Tue, Apr 02, 2024 at 09:40:08PM +0800, Dawei Li wrote:
> > Hi Mark,
> >
> > Thanks for the quick review.
> >
> > On Tue, Apr 02, 2024 at 12:12:50PM +0100, Mark Rutland wrote:
> > > On Tue, Apr 02, 2024 at 06:56:01PM +0800, Dawei Li wrote:
> > > > Hi,
> > > >
> > > > This series try to eliminate direct cpumask var allocation from stack
> > > > for perf subsystem.
> > > >
> > > > Direct/explicit allocation of cpumask on stack could be dangerous since
> > > > it can lead to stack overflow for systems with big NR_CPUS or
> > > > CONFIG_CPUMASK_OFFSTACK=y.
> > > >
> > > > For arm64, it's more urgent since commit 3fbd56f0e7c1 ("ARM64: Dynamically
> > > > allocate cpumasks and increase supported CPUs to 512").
> > > >
> > > > It's sort of a pattern that almost every cpumask var in perf subystem
> > > > occurs in teardown callback of cpuhp. In which case, if dynamic
> > > > allocation failed(which is unlikely), we choose return 0 rather than
> > > > -ENOMEM to caller cuz:
> > > > @teardown is not supposed to fail and if it does, system crashes:
> > >
> > > .. but we've left the system in an incorrect state, so that makes no sense.
> > >
> > > As I commented on the first patch, NAK to dynamically allocating cpumasks in
> > > the CPUHP callbacks. Please allocate the necessry cpumasks up-front when we
> > > probe the PMU. At that time we can handle an allocation failure by cleaning up
> > > and failing to probe the PMU, and then the CPUHP callbacks don't need to
> > > allocate memory to offline a CPU...
> >
> > Agreed that dynamically allocation in callbacks lead to inconsistency
> > to system.
> >
> > My (original)alternative plan is simple but ugly, just make cpumask var
> > _static_ and add extra static lock to protect it.
> >
> > The only difference between solution above and your proposal is static/
> > dynamic alloction. CPUHP's teardown cb is supposed to run in targetted
> > cpuhp thread for most cases, and it's racy. Even the cpumask var is
> > wrapped in dynamically allocated struct xxx_pmu, it's still shareable
> > between different threads/contexts and needs proper protection.
>
> I was under the impression that the cpuhp callbacks were *strictly* serialised.
> If that's not the case, the existing offlining callbacks are horrendously
> broken.
>
> Are you *certain* these can race?
>
> Regardless, adding additional locking here is not ok.
>
> > Simple as this(_untested_):
> >
> > diff --git a/drivers/perf/arm-cmn.c b/drivers/perf/arm-cmn.c
> > index 7ef9c7e4836b..fa89c3db4d7d 100644
> > --- a/drivers/perf/arm-cmn.c
> > +++ b/drivers/perf/arm-cmn.c
> > @@ -1950,18 +1950,24 @@ static int arm_cmn_pmu_offline_cpu(unsigned int cpu, struct hlist_node *cpuhp_no
> > struct arm_cmn *cmn;
> > unsigned int target;
> > int node;
> > - cpumask_t mask;
> > + static cpumask_t mask;
> > + static DEFINE_SPINLOCK(cpumask_lock);
> >
> > cmn = hlist_entry_safe(cpuhp_node, struct arm_cmn, cpuhp_node);
> > if (cpu != cmn->cpu)
> > return 0;
> >
> > + spin_lock(&cpumask_lock);
> > +
> > node = dev_to_node(cmn->dev);
> > if (cpumask_and(&mask, cpumask_of_node(node), cpu_online_mask) &&
> > cpumask_andnot(&mask, &mask, cpumask_of(cpu)))
> > target = cpumask_any(&mask);
> > else
> > target = cpumask_any_but(cpu_online_mask, cpu);
> > +
> > + spin_unlock(&cpumask_lock);
> > +
> > if (target < nr_cpu_ids)
> > arm_cmn_migrate(cmn, target);
> > return 0;
>
> Looking at this case, the only reason we need the mask is because it made the
> logic a little easier to write. All we really want is to choose some CPU in the
> intersection of two masks ignoring a specific CPU, and there was no helper
> function to do that.
>
> We can add a new helper to do that for us, which would avoid redundant work to
> manipulate the entire mask, and it would make the existing code simpler. I had
> a series a few years back to add cpumask_any_and_but():
>
> https://lore.kernel.org/lkml/1486381132-5610-1-git-send-email-mark.rutland@xxxxxxx/
Sounds a perfect idea!
Actually I am re-implementing new series on top of your seven-years-late-yet-still-helpful
patch, with minor update on it:
diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index 1c29947db848..121f3ac757ff 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -388,6 +388,29 @@ unsigned int cpumask_any_but(const struct cpumask *mask, unsigned int cpu)
return i;
}
+/**
+ * cpumask_any_and_but - pick a "random" cpu from *mask1 & *mask2, but not this one.
+ * @mask1: the first input cpumask
+ * @mask2: the second input cpumask
+ * @cpu: the cpu to ignore
+ *
+ * Returns >= nr_cpu_ids if no cpus set.
+ */
+static inline
+unsigned int cpumask_any_and_but(const struct cpumask *mask1,
+ const struct cpumask *mask2,
+ unsigned int cpu)
+{
+ unsigned int i;
+
+ cpumask_check(cpu);
+ i = cpumask_first_and(mask1, mask2);
+ if (i != cpu)
+ return i;
+
+ return cpumask_next_and(cpu, mask1, mask2);
+}
+
/**
* cpumask_nth - get the Nth cpu in a cpumask
* @srcp: the cpumask pointer
Change from your original version:
1 Moved to cpumask.h, just like other helpers.
2 Return value converted to unsigned int.
3 Remove EXPORT_SYMBOL, for obvious reason.
I will respin V2 as a whole as soon as possible.
Thanks,
Dawei
>
> .. and that's easy to use here, e.g.
>
> | diff --git a/drivers/perf/arm-cmn.c b/drivers/perf/arm-cmn.c
> | index 7ef9c7e4836b7..c6bbd387ccf8b 100644
> | --- a/drivers/perf/arm-cmn.c
> | +++ b/drivers/perf/arm-cmn.c
> | @@ -1950,17 +1950,15 @@ static int arm_cmn_pmu_offline_cpu(unsigned int cpu, struct hlist_node *cpuhp_no
> | struct arm_cmn *cmn;
> | unsigned int target;
> | int node;
> | - cpumask_t mask;
> |
> | cmn = hlist_entry_safe(cpuhp_node, struct arm_cmn, cpuhp_node);
> | if (cpu != cmn->cpu)
> | return 0;
> |
> | node = dev_to_node(cmn->dev);
> | - if (cpumask_and(&mask, cpumask_of_node(node), cpu_online_mask) &&
> | - cpumask_andnot(&mask, &mask, cpumask_of(cpu)))
> | - target = cpumask_any(&mask);
> | - else
> | + target = cpumask_any_and_but(cpu_online_mask, cpumask_of_node(node),
> | + cpu);
> | + if (target >= nr_cpu_ids)
> | target = cpumask_any_but(cpu_online_mask, cpu);
> | if (target < nr_cpu_ids)
> | arm_cmn_migrate(cmn, target);
>
> It doesn't trivially rebase since the cpumask code has changed a fair amount,
> but I've managed to do that locally, and I can send that out as a
> seven-years-late v2 if it's useful.
>
> From a quick scan, it looks like that'd handle all cases in this series. Are
> there any patterns in this series for which that would not be sufficient?
>
> Mark.
>
> >
> > And yes, static allocation is evil :)
> >
> >
> > Thanks,
> >
> > Dawei
> >
> > >
> > > Also, for the titles it'd be better to say something like "avoid placing
> > > cpumasks on the stack", because "explicit cpumask var allocation" sounds like
> > > the use of alloc_cpumask_var().
> >
> > Sound great! I will update it.
> >
> > >
> > > Mark.
> > >
> > > >
> > > > static int cpuhp_issue_call(int cpu, enum cpuhp_state state, bool bringup,
> > > > struct hlist_node *node)
> > > > {
> > > > struct cpuhp_step *sp = cpuhp_get_step(state);
> > > > int ret;
> > > >
> > > > /*
> > > > * If there's nothing to do, we done.
> > > > * Relies on the union for multi_instance.
> > > > */
> > > > if (cpuhp_step_empty(bringup, sp))
> > > > return 0;
> > > > /*
> > > > * The non AP bound callbacks can fail on bringup. On teardown
> > > > * e.g. module removal we crash for now.
> > > > */
> > > > #ifdef CONFIG_SMP
> > > > if (cpuhp_is_ap_state(state))
> > > > ret = cpuhp_invoke_ap_callback(cpu, state, bringup, node);
> > > > else
> > > > ret = cpuhp_invoke_callback(cpu, state, bringup, node,
> > > > NULL);
> > > > #else
> > > > ret = cpuhp_invoke_callback(cpu, state, bringup, node, NULL);
> > > > #endif
> > > > BUG_ON(ret && !bringup);
> > > > return ret;
> > > > }
> > > >
> > > > Dawei Li (9):
> > > > perf/alibaba_uncore_drw: Avoid explicit cpumask var allocation from
> > > > stack
> > > > perf/arm-cmn: Avoid explicit cpumask var allocation from stack
> > > > perf/arm_cspmu: Avoid explicit cpumask var allocation from stack
> > > > perf/arm_dsu: Avoid explicit cpumask var allocation from stack
> > > > perf/dwc_pcie: Avoid explicit cpumask var allocation from stack
> > > > perf/hisi_pcie: Avoid explicit cpumask var allocation from stack
> > > > perf/hisi_uncore: Avoid explicit cpumask var allocation from stack
> > > > perf/qcom_l2: Avoid explicit cpumask var allocation from stack
> > > > perf/thunder_x2: Avoid explicit cpumask var allocation from stack
> > > >
> > > > drivers/perf/alibaba_uncore_drw_pmu.c | 13 +++++++++----
> > > > drivers/perf/arm-cmn.c | 13 +++++++++----
> > > > drivers/perf/arm_cspmu/arm_cspmu.c | 13 +++++++++----
> > > > drivers/perf/arm_dsu_pmu.c | 18 +++++++++++++-----
> > > > drivers/perf/dwc_pcie_pmu.c | 17 +++++++++++------
> > > > drivers/perf/hisilicon/hisi_pcie_pmu.c | 15 ++++++++++-----
> > > > drivers/perf/hisilicon/hisi_uncore_pmu.c | 13 +++++++++----
> > > > drivers/perf/qcom_l2_pmu.c | 15 ++++++++++-----
> > > > drivers/perf/thunderx2_pmu.c | 20 ++++++++++++--------
> > > > 9 files changed, 92 insertions(+), 45 deletions(-)
> > > >
> > > >
> > > > Thanks,
> > > >
> > > > Dawei
> > > >
> > > > --
> > > > 2.27.0
> > > >
> > >
>