* Gautham R Shenoy <[email protected]> wrote:
> void lock_cpu_hotplug(void)
> {
> + DECLARE_WAITQUEUE(wait, current);
> + spin_lock(&cpu_hotplug.lock);
> + cpu_hotplug.reader_count++;
this should be per-CPU - lock_cpu_hotplug() should _not_ be a globally
synchronized event.
CPU removal is such a rare event that we can easily do something like a
global read-mostly 'CPU is locked for writes' flag (plus a completion
queue) that the 'write' side takes atomically - combined with per-CPU
refcount and a waitqueue that the read side increases/decreases and
wakes. Read-locking of the CPU is much more common and should be
fundamentally scalable: it should increase the per-CPU refcount, then
check the global 'writer active' flag, and if the writer flag is set, it
should wait on the global completion queue. When a reader drops the
refcount it should wake up the per-CPU waitqueue. [in which a writer
might be waiting for the refcount to go down to 0.]
Ingo
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