Re: [Qemu-devel] [PATCH V6 08/10] migration: calculate vCPU blocktime on dst side

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Alexey (a.perevalov@samsung.com) wrote:
> On Wed, May 24, 2017 at 03:53:05PM +0800, Peter Xu wrote:
> > On Tue, May 23, 2017 at 02:31:09PM +0300, Alexey Perevalov wrote:
> > > This patch provides blocktime 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 blocktime per vCPU (could be traced with page_fault_addr)
> > > 
> > > Blocktime will not calculated if postcopy_blocktime field of
> > > MigrationIncomingState wasn't initialized.
> > > 
> > > Signed-off-by: Alexey Perevalov <a.perevalov@samsung.com>
> > > ---
> > >  migration/postcopy-ram.c | 102 ++++++++++++++++++++++++++++++++++++++++++++++-
> > >  migration/trace-events   |   5 ++-
> > >  2 files changed, 105 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> > > index d647769..e70c44b 100644
> > > --- a/migration/postcopy-ram.c
> > > +++ b/migration/postcopy-ram.c
> > > @@ -23,6 +23,7 @@
> > >  #include "postcopy-ram.h"
> > >  #include "sysemu/sysemu.h"
> > >  #include "sysemu/balloon.h"
> > > +#include <sys/param.h>
> > >  #include "qemu/error-report.h"
> > >  #include "trace.h"
> > >  
> > > @@ -577,6 +578,101 @@ 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;
> > > +}
> > > +
> > > +static void mark_postcopy_blocktime_begin(uint64_t addr, uint32_t ptid,
> > > +        RAMBlock *rb)
> > > +{
> > > +    int cpu;
> > > +    unsigned long int nr_bit;
> > > +    MigrationIncomingState *mis = migration_incoming_get_current();
> > > +    PostcopyBlocktimeContext *dc = mis->blocktime_ctx;
> > > +    int64_t now_ms;
> > > +
> > > +    if (!dc || ptid == 0) {
> > > +        return;
> > > +    }
> > > +    cpu = get_mem_fault_cpu_index(ptid);
> > > +    if (cpu < 0) {
> > > +        return;
> > > +    }
> > > +    nr_bit = get_copied_bit_offset(addr);
> > > +    if (test_bit(nr_bit, mis->copied_pages)) {
> > > +        return;
> > > +    }
> > > +    now_ms = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
> > > +    if (dc->vcpu_addr[cpu] == 0) {
> > > +        atomic_inc(&dc->smp_cpus_down);
> > > +    }
> > > +
> > > +    atomic_xchg__nocheck(&dc->vcpu_addr[cpu], addr);
> > > +    atomic_xchg__nocheck(&dc->last_begin, now_ms);
> > > +    atomic_xchg__nocheck(&dc->page_fault_vcpu_time[cpu], now_ms);
> > 
> > Looks like this is not what you and Dave have discussed?
> > 
> > (Btw, sorry to have not followed the thread recently, so I just went
> >  over the discussion again...)
> > 
> > What I see that Dave suggested is (I copied from Dave's email):
> > 
> > blocktime_start:
> >    set CPU stall address
> >    check bitmap entry
> >      if set then zero stall-address
> > 
> > While here it is:
> > 
> > blocktime_start:
> >    check bitmap entry
> >      if set then return
> >    set CPU stall address
> > 
> > I don't think current version can really solve the risk condition. See
> > this possible sequence:
> > 
> >        receive-thread             fault-thread 
> >        --------------             ------------
> >                                   blocktime_start
> >                                     check bitmap entry,
> >                                       if set then return
> >        blocktime_end
> >          set bitmap entry
> >          read CPU stall address,
> >            if none-0 then zero it
> >                                     set CPU stall address [1]
> >         
> > Then imho the address set at [1] will be stall again until forever.
> > 
> agree, I check is in incorrect order
> 
> > I think we should follow exactly what Dave has suggested.
> > 
> > And.. after a second thought, I am afraid even this would not satisfy
> > all risk conditions. What if we consider the UFFDIO_COPY ioctl in?
> > AFAIU after UFFDIO_COPY the faulted vcpu can be running again, then
> > the question is, can it quickly trigger another page fault?
> >
> yes, it can
> 
> > Firstly, a workable sequence is (adding UFFDIO_COPY ioctl in, and
> > showing vcpu-thread X as well):
> > 
> >   receive-thread       fault-thread        vcpu-thread X
> >   --------------       ------------        -------------
> >                                            fault at addr A1
> >                        fault_addr[X]=A1
> >   UFFDIO_COPY page A1
> >   check fault_addr[X] with A1
> >     if match, clear fault_addr[X]
> >                                            vcpu X starts
> > 
> > This is fine.
> > 
> > While since "vcpu X starts" can be right after UFFDIO_COPY, can this
> > be possible?
> Previous picture isn't possible, due to mark_postcopy_blocktime_end
> is being called right after ioctl, and vCPU is waking up
> inside ioctl, so check fault_addr will be after vcpu X starts.

It's the optimistic view when you get lucky.

> > 
> >   receive-thread       fault-thread        vcpu-thread X
> >   --------------       ------------        -------------
> >                                            fault at addr A1
> >                        fault_addr[X]=A1
> >   UFFDIO_COPY page A1
> >                                            vcpu X starts
> >                                            fault at addr A2
> >                        fault_addr[X]=A2
> >   check fault_addr[X] with A1
> >     if match, clear fault_addr[X]
> >         ^
> >         |
> >         +---------- here it will not match since now fault_addr[X]==A2
> > 
> > Then looks like fault_addr[X], which is currently A1, will stall
> > again?
> 
> It will be another address(A2), but probably the same vCPU and if in
> this case blocktime_start will be called before blocktime_end we lost
> block time for page A1. Address of the page is unique key in this
> process, not vCPU ;)
> Here maybe reasonable to wake up vCPU after blocktime_end.

Yes, that probably works better.
This ordering is really tricky!

One thing you might want to look for, I suspect there are cases
where you get faults from vCPUs but then it carries on running
doing something else because of KVM asynchronous page faults.
I'm not sure what you end up with when that happens.

Dave

> 
> 
> 
> > 
> > (I feel like finally we may need something like a per-cpu lock... or I
> >  must have missed something)
> I think no, because locking time of the vCPU is critical in this process.
> > 
> > > +
> > > +    trace_mark_postcopy_blocktime_begin(addr, dc, dc->page_fault_vcpu_time[cpu],
> > > +            cpu);
> > > +}
> > > +
> > > +static void mark_postcopy_blocktime_end(uint64_t addr)
> > > +{
> > > +    MigrationIncomingState *mis = migration_incoming_get_current();
> > > +    PostcopyBlocktimeContext *dc = mis->blocktime_ctx;
> > > +    int i, affected_cpu = 0;
> > > +    int64_t now_ms;
> > > +    bool vcpu_total_blocktime = false;
> > > +    unsigned long int nr_bit;
> > > +
> > > +    if (!dc) {
> > > +        return;
> > > +    }
> > > +    /* mark that page as copied */
> > > +    nr_bit = get_copied_bit_offset(addr);
> > > +    set_bit_atomic(nr_bit, mis->copied_pages);
> > > +
> > > +    now_ms = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
> > > +
> > > +    /* lookup cpu, to clear it,
> > > +     * that algorithm looks straighforward, but it's not
> > > +     * optimal, more optimal algorithm is keeping tree or hash
> > > +     * where key is address value is a list of  */
> > > +    for (i = 0; i < smp_cpus; i++) {
> > > +        uint64_t vcpu_blocktime = 0;
> > > +        if (atomic_fetch_add(&dc->vcpu_addr[i], 0) != addr) {
> > > +            continue;
> > > +        }
> > > +        atomic_xchg__nocheck(&dc->vcpu_addr[i], 0);
> > 
> > Why use *__nocheck() rather than atomic_xchg() or even atomic_read()?
> > Same thing happened a lot in current patch.
> atomic_read/atomic_xchg for mingw/(gcc on arm32) has build time check,
> 
> QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *));
> 
> it prevents using 64 atomic operation on 32 architecture, just mingw I
> think, but postcopy-ram.c isn't compiling for mingw.
> On other 32 platforms as I know clang/gcc allow to use 8 bytes
> long variables in built atomic operations. In arm32 it allows in
> builtin. But QEMU on arm32 still
> has that sanity check, and I think it's bug, so I just worked it around.
> Maybe better was to fix it.
> 
> I tested in docker, using follow command:
> make docker-test-build@debian-armhf-cross
> 
> And got following error
> 
> /tmp/qemu-test/src/migration/postcopy-ram.c: In function
> 'mark_postcopy_blocktime_begin':
> /tmp/qemu-test/src/include/qemu/compiler.h:86:30: error: static
> assertion failed: "not expecting: sizeof(*&dc->vcpu_addr[cpu]) >
> sizeof(void *)"
>  #define QEMU_BUILD_BUG_ON(x) _Static_assert(!(x), "not expecting: " #x)
> 
> when I used atomic_xchg,
> I agree with you, but I think need to fix atomic.h firstly and add additional
> #ifdef there.
> 
> And I didn't want to split 64 bit values onto 32 bit values, but I saw
> in mailing list people are doing it.
> 
> > 
> > Thanks,
> > 
> > > +        vcpu_blocktime = now_ms -
> > > +            atomic_fetch_add(&dc->page_fault_vcpu_time[i], 0);
> > > +        affected_cpu += 1;
> > > +        /* we need to know is that mark_postcopy_end was due to
> > > +         * faulted page, another possible case it's prefetched
> > > +         * page and in that case we shouldn't be here */
> > > +        if (!vcpu_total_blocktime &&
> > > +            atomic_fetch_add(&dc->smp_cpus_down, 0) == smp_cpus) {
> > > +            vcpu_total_blocktime = true;
> > > +        }
> > > +        /* continue cycle, due to one page could affect several vCPUs */
> > > +        dc->vcpu_blocktime[i] += vcpu_blocktime;
> > > +    }
> > > +
> > > +    atomic_sub(&dc->smp_cpus_down, affected_cpu);
> > > +    if (vcpu_total_blocktime) {
> > > +        dc->total_blocktime += now_ms - atomic_fetch_add(&dc->last_begin, 0);
> > > +    }
> > > +    trace_mark_postcopy_blocktime_end(addr, dc, dc->total_blocktime);
> > > +}
> > > +
> > >  /*
> > >   * Handle faults detected by the USERFAULT markings
> > >   */
> > > @@ -654,8 +750,11 @@ 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_blocktime_begin((uintptr_t)(msg.arg.pagefault.address),
> > > +                msg.arg.pagefault.feat.ptid, rb);
> > >          /*
> > >           * Send the request to the source - we want to request one
> > >           * of our host page sizes (which is >= TPS)
> > > @@ -750,6 +849,7 @@ int postcopy_place_page(MigrationIncomingState *mis, void *host, void *from,
> > >  
> > >          return -e;
> > >      }
> > > +    mark_postcopy_blocktime_end((uint64_t)(uintptr_t)host);
> > >  
> > >      trace_postcopy_place_page(host);
> > >      return 0;
> > > diff --git a/migration/trace-events b/migration/trace-events
> > > index 5b8ccf3..7bdadbb 100644
> > > --- a/migration/trace-events
> > > +++ b/migration/trace-events
> > > @@ -112,6 +112,8 @@ 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_blocktime_begin(uint64_t addr, void *dd, int64_t time, int cpu) "addr 0x%" PRIx64 " dd %p time %" PRId64 " cpu %d"
> > > +mark_postcopy_blocktime_end(uint64_t addr, void *dd, int64_t time) "addr 0x%" PRIx64 " dd %p time %" PRId64
> > >  
> > >  # migration/rdma.c
> > >  qemu_rdma_accept_incoming_migration(void) ""
> > > @@ -188,7 +190,7 @@ 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) ""
> > > @@ -197,6 +199,7 @@ 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"
> > > -- 
> > > 1.8.3.1
> > > 
> > 
> > -- 
> > Peter Xu
> > 
> 
> -- 
> 
> BR
> Alexey
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK