On 2019-01-08 16:16, Davidlohr Bueso wrote:
On 2019-01-08 04:42, Roman Penyaev wrote:
What we can do:
a) disable irqs if we are not in interrupt.
b) revert the patch completely.
David, is it really crucial in terms of performance to avoid double
local_irq_save() on Xen on this ep_poll_callback() hot path?
Note that such optimizations are also relevant for baremetal, ie: x86
PUSHF + POPF can be pretty expensive because of insn dependencies.
For example why not to do the following:
if (!in_interrupt())
local_irq_save(flags);
read_lock(ep->lock);
with huge comment explaining performance number.
Or just give up and simply revert the original patch completely
and always call read_lock_irqsave().
Yeah so the reason why I had done the other epoll lock irq
optimizations was because they were painfully obvious.
ep_poll_callback(), however is a different beast, as you've
encountered. I vote for not shooting ourselves in the foot and just
dropping this patch -- most large performance benefits will come from
microbenches anyway.
Then I will send another patch which changes read_lock() on
read_lock_irqsave(),
since simple revert of the original patch won't work.
Thanks.
--
Roman
