Messages in this thread | | | Subject | Re: Is stopmachine() preempt safe? | From | Rusty Russell <> | Date | Mon, 28 Aug 2006 19:21:18 +1000 |
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On Mon, 2006-08-28 at 16:36 +1000, Keith Owens wrote: > There is a lot of code in the kernel that runs cpu_online_map without > taking any locks and without disabling preemption. Obviously we do not > want all that code to lock or disable preemption, it will kill > scalability.
There is actually not much code which should use cpu_online_map. Code which does must be careful: you generally need to think about handling cpu hotplug notifiers as well as the map changing underneath you.
Doing a brief audit, ignoring the already-acknowledged cpufreq code and arch-specifics, I can see these cases which seem suspicious:
./drivers/acpi/processor_core.c:acpi_processor_handle_eject() - I assume this is relying on some other mechanism so the cpu doesn't get onlined? - A couple of other num_online_cpus() there in ACPI might need a rethink for hotplug CPU though.
./kernel/irq/proc.c: irq_affinity_write_proc() - seems complicated, but I think migration.c handles when cpus gone offline?
./drivers/oprofile/cpu_buffer.c: - needs to handle hotplug cpus (or just say don't do that?)
./drivers/infiniband/hw/ipath/ipath_file_ops.c: - seems to be using num_online_cpus as a really poor heuristic, and incorrectly (for i = 0; i < num_online_cpus(); i++) <-- i is not a valid CPU number!).
./kernel/power/main.c:suspend_prepare() - suspicious, code here, too.
./net/core/dev.c: net_dma_rebalance() - This is a heuristic, which may be OK. ./net/core/dev.c: softnet_get_online() - It'd be nice if net/dev/core.c used cpu_possible() not cpu_online() to report stats, so they don't get lost from offlined CPUs.
./net/core/pktgen.c: pg_init() - Assumes no CPU plugging, but is a pretty specialized driver.
(Other uses get away with being in initcalls, or on platforms without hotplug CPU).
Disappointingly, none of these would be fixed by changing the semantics of stop_machine; they rely on the online cpus and must take action when they change, whether they are reading the online_cpu_map at the time or not.
Rusty. -- Help! Save Australia from the worst of the DMCA: http://linux.org.au/law
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