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

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

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.

> 
>   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.



> 
> (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