Re: [Xen-devel] Performance overhead of paravirt_ops on nativeidentified
From: Jeremy Fitzhardinge
Date: Fri May 15 2009 - 14:51:33 EST
Jan Beulich wrote:
A patch for the pv-ops kernel would require some time. What I can give you
right away - just for reference - are the sources we currently use in our kernel:
attached.
Hm, I see. Putting a call out to a pv-ops function in the ticket lock
slow path looks pretty straightforward. The need for an extra lock on
the contended unlock side is a bit unfortunate; have you measured to see
what hit that has? Seems to me like you could avoid the problem by
using per-cpu storage rather than stack storage (though you'd need to
copy the per-cpu data to stack when handling a nested spinlock).
What's the thinking behind the xen_spin_adjust() stuff?
static __always_inline void __ticket_spin_lock(raw_spinlock_t *lock) {
unsigned int token, count; bool free; __ticket_spin_lock_preamble; if
(unlikely(!free)) token = xen_spin_adjust(lock, token); do { count = 1
<< 10; __ticket_spin_lock_body; } while (unlikely(!count) &&
!xen_spin_wait(lock, token)); }
How does this work? Doesn't it always go into the slowpath loop even if
the preamble got the lock with no contention?
J
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