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

[Alexey <a.perevalov@samsung.com>]

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 <a.perevalov@samsung.com>
> > >>---
> > >>  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.

> > > Can it be some other page? I
> > >don't know. But since we have the loop below to make sure of it, why
> > >not?

> > 
> > >A nitpick on perf: when there are lots of vcpus, the algo might be
> > >slow since we have several places that loops over the smp_vcpus. But
> > >this can be totally future work on top, and current way is good enough
> > >at least for me.
> > 
> > >(for the nit: maybe add a hash, key=thread_id, value=cpu_index, then
> > >  get_mem_fault_cpu_index() can be faster using the hash; meanwhile
> > >  keep a counter A of page faulted vcpus, use atomic ops with it, then
> > >  here all_vcpu_down can be checked by A == smp_vcpus)
> > just binary search in get_mem_fault_cpu_index will be nice too )
> > also, it's good idea to keep all_vcpu_down in PostcopyDowntimeContext.
> > 
> > >
> > >Thanks,
> > >
> > >>+    }
> > >>+
> > >>+    /* lookup cpu, to clear it */
> > >>+    for (i = 0; i < smp_cpus; i++) {
> > >>+        uint64_t vcpu_downtime;
> > >>+
> > >>+        if (dc->vcpu_addr[i] != addr) {
> > >>+            continue;
> > >>+        }
> > >>+
> > >>+        vcpu_downtime = now - dc->page_fault_vcpu_time[i];
> > >>+
> > >>+        dc->vcpu_addr[i] = 0;
> > >>+        dc->vcpu_downtime[i] += vcpu_downtime;
> > >>+    }
> > >>+
> > >>+    trace_mark_postcopy_downtime_end(addr, dc, dc->total_downtime);
> > >>+}
> > >>+
> > >>+/*
> > >>+ * This function just provide calculated before downtime per cpu and trace it.
> > >>+ * Total downtime is calculated in mark_postcopy_downtime_end.
> > >>+ *
> > >>+ *
> > >>+ * Assume we have 3 CPU
> > >>+ *
> > >>+ *      S1        E1           S1               E1
> > >>+ * -----***********------------xxx***************------------------------> CPU1
> > >>+ *
> > >>+ *             S2                E2
> > >>+ * ------------****************xxx---------------------------------------> CPU2
> > >>+ *
> > >>+ *                         S3            E3
> > >>+ * ------------------------****xxx********-------------------------------> CPU3
> > >>+ *
> > >>+ * We have sequence S1,S2,E1,S3,S1,E2,E3,E1
> > >>+ * S2,E1 - doesn't match condition due to sequence S1,S2,E1 doesn't include CPU3
> > >>+ * S3,S1,E2 - sequence includes all CPUs, in this case overlap will be S1,E2 -
> > >>+ *            it's a part of total downtime.
> > >>+ * S1 - here is last_begin
> > >>+ * Legend of the picture is following:
> > >>+ *              * - means downtime per vCPU
> > >>+ *              x - means overlapped downtime (total downtime)
> > >>+ */
> > >>+uint64_t get_postcopy_total_downtime(void)
> > >>+{
> > >>+    MigrationIncomingState *mis = migration_incoming_get_current();
> > >>+
> > >>+    if (!mis->downtime_ctx) {
> > >>+        return 0;
> > >>+    }
> > >>+
> > >>+    if (trace_event_get_state(TRACE_DOWNTIME_PER_CPU)) {
> > >>+        int i;
> > >>+        for (i = 0; i < smp_cpus; i++) {
> > >>+            trace_downtime_per_cpu(i, mis->downtime_ctx->vcpu_downtime[i]);
> > >>+        }
> > >>+    }
> > >>+    return mis->downtime_ctx->total_downtime;
> > >>+}
> > >>diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> > >>index f3688f5..cf2b935 100644
> > >>--- a/migration/postcopy-ram.c
> > >>+++ b/migration/postcopy-ram.c
> > >>@@ -23,6 +23,7 @@
> > >>  #include "migration/postcopy-ram.h"
> > >>  #include "sysemu/sysemu.h"
> > >>  #include "sysemu/balloon.h"
> > >>+#include <sys/param.h>
> > >>  #include "qemu/error-report.h"
> > >>  #include "trace.h"
> > >>@@ -468,6 +469,19 @@ static int ram_block_enable_notify(const char *block_name, void *host_addr,
> > >>      return 0;
> > >>  }
> > >>+static int get_mem_fault_cpu_index(uint32_t pid)
> > >>+{
> > >>+    CPUState *cpu_iter;
> > >>+
> > >>+    CPU_FOREACH(cpu_iter) {
> > >>+        if (cpu_iter->thread_id == pid) {
> > >>+            return cpu_iter->cpu_index;
> > >>+        }
> > >>+    }
> > >>+    trace_get_mem_fault_cpu_index(pid);
> > >>+    return -1;
> > >>+}
> > >>+
> > >>  /*
> > >>   * Handle faults detected by the USERFAULT markings
> > >>   */
> > >>@@ -545,8 +559,11 @@ static void *postcopy_ram_fault_thread(void *opaque)
> > >>          rb_offset &= ~(qemu_ram_pagesize(rb) - 1);
> > >>          trace_postcopy_ram_fault_thread_request(msg.arg.pagefault.address,
> > >>                                                  qemu_ram_get_idstr(rb),
> > >>-                                                rb_offset);
> > >>+                                                rb_offset,
> > >>+                                                msg.arg.pagefault.feat.ptid);
> > >>+        mark_postcopy_downtime_begin((uintptr_t)(msg.arg.pagefault.address),
> > >>+                         get_mem_fault_cpu_index(msg.arg.pagefault.feat.ptid));
> > >>          /*
> > >>           * Send the request to the source - we want to request one
> > >>           * of our host page sizes (which is >= TPS)
> > >>@@ -641,6 +658,7 @@ int postcopy_place_page(MigrationIncomingState *mis, void *host, void *from,
> > >>          return -e;
> > >>      }
> > >>+    mark_postcopy_downtime_end((uint64_t)host);
> > >>      trace_postcopy_place_page(host);
> > >>      return 0;
> > >>diff --git a/migration/trace-events b/migration/trace-events
> > >>index b8f01a2..d338810 100644
> > >>--- a/migration/trace-events
> > >>+++ b/migration/trace-events
> > >>@@ -110,6 +110,9 @@ process_incoming_migration_co_end(int ret, int ps) "ret=%d postcopy-state=%d"
> > >>  process_incoming_migration_co_postcopy_end_main(void) ""
> > >>  migration_set_incoming_channel(void *ioc, const char *ioctype) "ioc=%p ioctype=%s"
> > >>  migration_set_outgoing_channel(void *ioc, const char *ioctype, const char *hostname)  "ioc=%p ioctype=%s hostname=%s"
> > >>+mark_postcopy_downtime_begin(uint64_t addr, void *dd, int64_t time, int cpu) "addr 0x%" PRIx64 " dd %p time %" PRId64 " cpu %d"
> > >>+mark_postcopy_downtime_end(uint64_t addr, void *dd, int64_t time) "addr 0x%" PRIx64 " dd %p time %" PRId64
> > >>+downtime_per_cpu(int cpu_index, int64_t downtime) "downtime cpu[%d]=%" PRId64
> > >>  # migration/rdma.c
> > >>  qemu_rdma_accept_incoming_migration(void) ""
> > >>@@ -186,7 +189,7 @@ postcopy_ram_enable_notify(void) ""
> > >>  postcopy_ram_fault_thread_entry(void) ""
> > >>  postcopy_ram_fault_thread_exit(void) ""
> > >>  postcopy_ram_fault_thread_quit(void) ""
> > >>-postcopy_ram_fault_thread_request(uint64_t hostaddr, const char *ramblock, size_t offset) "Request for HVA=%" PRIx64 " rb=%s offset=%zx"
> > >>+postcopy_ram_fault_thread_request(uint64_t hostaddr, const char *ramblock, size_t offset, uint32_t pid) "Request for HVA=%" PRIx64 " rb=%s offset=%zx %u"
> > >>  postcopy_ram_incoming_cleanup_closeuf(void) ""
> > >>  postcopy_ram_incoming_cleanup_entry(void) ""
> > >>  postcopy_ram_incoming_cleanup_exit(void) ""
> > >>@@ -195,6 +198,7 @@ save_xbzrle_page_skipping(void) ""
> > >>  save_xbzrle_page_overflow(void) ""
> > >>  ram_save_iterate_big_wait(uint64_t milliconds, int iterations) "big wait: %" PRIu64 " milliseconds, %d iterations"
> > >>  ram_load_complete(int ret, uint64_t seq_iter) "exit_code %d seq iteration %" PRIu64
> > >>+get_mem_fault_cpu_index(uint32_t pid) "pid %u is not vCPU"
> > >>  # migration/exec.c
> > >>  migration_exec_outgoing(const char *cmd) "cmd=%s"
> > >>-- 
> > >>1.9.1
> > >>
> > 
> > 
> > -- 
> > Best regards,
> > Alexey Perevalov
> 
> -- 
> Peter Xu
> 

-- 

BR
Alexey