Re: [PATCH RFC v3 1/6] x86/paravirt: Add pv_idle_ops to paravirt ops

On 13/11/17 11:06, Quan Xu wrote:

From: Quan Xu <redacted>

So far, pv_idle_ops.poll is the only ops for pv_idle. .poll is called
in idle path which will poll for a while before we enter the real idle
state.

In virtualization, idle path includes several heavy operations
includes timer access(LAPIC timer or TSC deadline timer) which will
hurt performance especially for latency intensive workload like message
passing task. The cost is mainly from the vmexit which is a hardware
context switch between virtual machine and hypervisor. Our solution is
to poll for a while and do not enter real idle path if we can get the
schedule event during polling.

Poll may cause the CPU waste so we adopt a smart polling mechanism to
reduce the useless poll.

Signed-off-by: Yang Zhang <redacted>
Signed-off-by: Quan Xu <redacted>
Cc: Juergen Gross <jgross@suse.com>
Cc: Alok Kataria <redacted>
Cc: Rusty Russell <redacted>
Cc: Thomas Gleixner <redacted>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: virtualization@lists.linux-foundation.org
Cc: linux-kernel@vger.kernel.org
Cc: xen-devel@lists.xenproject.org

Hmm, is the idle entry path really so critical to performance that a new
pvops function is necessary? Wouldn't a function pointer, maybe guarded
by a static key, be enough? A further advantage would be that this would
work on other architectures, too.


Juergen