Re: [Qemu-devel] [PATCH RESEND V3 5/6] migration: calculate downtime on dst side

[Peter Xu]

On Mon, May 08, 2017 at 12:08:07PM +0300, Alexey wrote:
> On Mon, May 08, 2017 at 02:29:06PM +0800, Peter Xu wrote:
> > On Fri, Apr 28, 2017 at 02:11:19PM +0300, Alexey Perevalov wrote:
> > > On 04/28/2017 01:00 PM, Peter Xu wrote:
> > > >On Fri, Apr 28, 2017 at 09:57:37AM +0300, Alexey Perevalov wrote:
> > > >>This patch provides downtime calculation per vCPU,
> > > >>as a summary and as a overlapped value for all vCPUs.
> > > >>
> > > >>This approach was suggested by Peter Xu, as an improvements of
> > > >>previous approch where QEMU kept tree with faulted page address and 
> > > >>cpus bitmask
> > > >>in it. Now QEMU is keeping array with faulted page address as value and 
> > > >>vCPU
> > > >>as index. It helps to find proper vCPU at UFFD_COPY time. Also it keeps
> > > >>list for downtime per vCPU (could be traced with page_fault_addr)
> > > >>
> > > >>For more details see comments for get_postcopy_total_downtime
> > > >>implementation.
> > > >>
> > > >>Downtime will not calculated if postcopy_downtime field of
> > > >>MigrationIncomingState wasn't initialized.
> > > >>
> > > >>Signed-off-by: Alexey Perevalov <address@hidden>
> > > >>---
> > > >>  include/migration/migration.h |   3 ++
> > > >>  migration/migration.c         | 103 
> > > >> ++++++++++++++++++++++++++++++++++++++++++
> > > >>  migration/postcopy-ram.c      |  20 +++++++-
> > > >>  migration/trace-events        |   6 ++-
> > > >>  4 files changed, 130 insertions(+), 2 deletions(-)
> > > >>
> > > >>diff --git a/include/migration/migration.h 
> > > >>b/include/migration/migration.h
> > > >>index e8fb68f..a22f9ce 100644
> > > >>--- a/include/migration/migration.h
> > > >>+++ b/include/migration/migration.h
> > > >>@@ -139,6 +139,9 @@ void migration_incoming_state_destroy(void);
> > > >>   * Functions to work with downtime context
> > > >>   */
> > > >>  struct DowntimeContext *downtime_context_new(void);
> > > >>+void mark_postcopy_downtime_begin(uint64_t addr, int cpu);
> > > >>+void mark_postcopy_downtime_end(uint64_t addr);
> > > >>+uint64_t get_postcopy_total_downtime(void);
> > > >>  struct MigrationState
> > > >>  {
> > > >>diff --git a/migration/migration.c b/migration/migration.c
> > > >>index ec76e5c..2c6f150 100644
> > > >>--- a/migration/migration.c
> > > >>+++ b/migration/migration.c
> > > >>@@ -2150,3 +2150,106 @@ PostcopyState postcopy_state_set(PostcopyState 
> > > >>new_state)
> > > >>      return atomic_xchg(&incoming_postcopy_state, new_state);
> > > >>  }
> > > >>+void mark_postcopy_downtime_begin(uint64_t addr, int cpu)
> > > >>+{
> > > >>+    MigrationIncomingState *mis = migration_incoming_get_current();
> > > >>+    DowntimeContext *dc;
> > > >>+    if (!mis->downtime_ctx || cpu < 0) {
> > > >>+        return;
> > > >>+    }
> > > >>+    dc = mis->downtime_ctx;
> > > >>+    dc->vcpu_addr[cpu] = addr;
> > > >>+    dc->last_begin = dc->page_fault_vcpu_time[cpu] =
> > > >>+        qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
> > > >>+
> > > >>+    trace_mark_postcopy_downtime_begin(addr, dc, 
> > > >>dc->page_fault_vcpu_time[cpu],
> > > >>+            cpu);
> > > >>+}
> > > >>+
> > > >>+void mark_postcopy_downtime_end(uint64_t addr)
> > > >>+{
> > > >>+    MigrationIncomingState *mis = migration_incoming_get_current();
> > > >>+    DowntimeContext *dc;
> > > >>+    int i;
> > > >>+    bool all_vcpu_down = true;
> > > >>+    int64_t now;
> > > >>+
> > > >>+    if (!mis->downtime_ctx) {
> > > >>+        return;
> > > >>+    }
> > > >>+    dc = mis->downtime_ctx;
> > > >>+    now = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
> > > >>+
> > > >>+    /* check all vCPU down,
> > > >>+     * QEMU has bitmap.h, but even with bitmap_and
> > > >>+     * will be a cycle */
> > > >>+    for (i = 0; i < smp_cpus; i++) {
> > > >>+        if (dc->vcpu_addr[i]) {
> > > >>+            continue;
> > > >>+        }
> > > >>+        all_vcpu_down = false;
> > > >>+        break;
> > > >>+    }
> > > >>+
> > > >>+    if (all_vcpu_down) {
> > > >>+        dc->total_downtime += now - dc->last_begin;
> > > >Shall we do this accouting only if we are sure the copied page address
> > > >is one of the page faulted addresses? Can it be some other page? I
> > > >don't know. But since we have the loop below to make sure of it, why
> > > >not?
> > > no, the downtime implies since page fault till the
> > > page will be copied.
> > > Yes another pages could be copied as well as pagefaulted,
> > > and they are copied due to prefetching, but it's not a downtime.
> > 
> > Not sure I got the point... Do you mean that when reach here, then
> > this page address is definitely one of the faulted addresses? I am not
> > 100% sure of this, but if you are sure, I am okay with it.
> Let me clarify.
> 
> > > >Shall we do this accouting only if we are sure the copied page address
> > > >is one of the page faulted addresses?
> Yes it's primary condition, due to there are could be another pages,
> which weren't faulted, they just was sent from source to destination,
> I called it prefetching.
> 
> I think I got why did you ask that question, because in this version
> all_vcpu_down and as a result total_downtime calculated incorrectly,
> it calculates every time when any page is copied, but it should
> be calculated only when faulted page copied, so only dc->vcpu_downtime
> was correctly calculated.

Exactly. I am afraid if we have such "prefetching" stuff then
total_downtime will be more than its real value.

-- 
Peter Xu