Re: [Qemu-devel] [PATCH v5 26/45] Postcopy page-map-incoming (PMI) structure

[David Gibson <david@gibson.dropbear.id.au>]

[-- Attachment #1: Type: text/plain, Size: 14490 bytes --]

On Wed, Feb 25, 2015 at 04:51:49PM +0000, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> 
> The PMI holds the state of each page on the incoming side,
> so that we can tell if the page is missing, already received
> or there is a request outstanding for it.
> 
> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> ---
>  include/migration/migration.h    |  18 ++++
>  include/migration/postcopy-ram.h |  12 +++
>  include/qemu/typedefs.h          |   1 +
>  migration/postcopy-ram.c         | 223 +++++++++++++++++++++++++++++++++++++++
>  4 files changed, 254 insertions(+)
> 
> diff --git a/include/migration/migration.h b/include/migration/migration.h
> index b44b9b2..86200b9 100644
> --- a/include/migration/migration.h
> +++ b/include/migration/migration.h
> @@ -48,6 +48,23 @@ enum mig_rpcomm_cmd {
>      MIG_RP_CMD_PONG,         /* Response to a PING; data (seq: be32 ) */
>  };
>  
> +/* Postcopy page-map-incoming - data about each page on the inbound side */
> +typedef enum {
> +   POSTCOPY_PMI_MISSING    = 0, /* page hasn't yet been received */

This appears to be a 3 space indent instead of the usual 4.

> +   POSTCOPY_PMI_REQUESTED  = 1, /* Kernel asked for a page, not yet got it */
> +   POSTCOPY_PMI_RECEIVED   = 2, /* We've got the page */
> +} PostcopyPMIState;

TBH, I'm not sure this enum actually helps anything.  I wonder if
things might actually be cleared if you simply treat the received and
requested bitmaps separately.

> +struct PostcopyPMI {
> +    QemuMutex      mutex;
> +    unsigned long *state0;        /* Together with state1 form a */
> +    unsigned long *state1;        /* PostcopyPMIState */

The comments on the lines above don't appear to shed any light on anything.

> +    unsigned long  host_mask;     /* A mask with enough bits set to cover one
> +                                     host page in the PMI */
> +    unsigned long  host_bits;     /* The number of bits in the map representing
> +                                     one host page */

I find the host_bits name fairly confusing.  Maybe "tp_per_hp"?

> +};
> +
>  typedef QLIST_HEAD(, LoadStateEntry) LoadStateEntry_Head;
>  
>  typedef enum {
> @@ -69,6 +86,7 @@ struct MigrationIncomingState {
>  
>      QEMUFile *return_path;
>      QemuMutex      rp_mutex;    /* We send replies from multiple threads */
> +    PostcopyPMI    postcopy_pmi;
>  };
>  
>  MigrationIncomingState *migration_incoming_get_current(void);
> diff --git a/include/migration/postcopy-ram.h b/include/migration/postcopy-ram.h
> index d81934f..e93ee8a 100644
> --- a/include/migration/postcopy-ram.h
> +++ b/include/migration/postcopy-ram.h
> @@ -13,7 +13,19 @@
>  #ifndef QEMU_POSTCOPY_RAM_H
>  #define QEMU_POSTCOPY_RAM_H
>  
> +#include "migration/migration.h"
> +
>  /* Return true if the host supports everything we need to do postcopy-ram */
>  bool postcopy_ram_supported_by_host(void);
>  
> +/*
> + * In 'advise' mode record that a page has been received.
> + */
> +void postcopy_hook_early_receive(MigrationIncomingState *mis,
> +                                 size_t bitmap_index);
> +
> +void postcopy_pmi_destroy(MigrationIncomingState *mis);
> +void postcopy_pmi_discard_range(MigrationIncomingState *mis,
> +                                size_t start, size_t npages);
> +void postcopy_pmi_dump(MigrationIncomingState *mis);
>  #endif
> diff --git a/include/qemu/typedefs.h b/include/qemu/typedefs.h
> index 611db46..924eeb6 100644
> --- a/include/qemu/typedefs.h
> +++ b/include/qemu/typedefs.h
> @@ -61,6 +61,7 @@ typedef struct PCIExpressHost PCIExpressHost;
>  typedef struct PCIHostState PCIHostState;
>  typedef struct PCMCIACardState PCMCIACardState;
>  typedef struct PixelFormat PixelFormat;
> +typedef struct PostcopyPMI PostcopyPMI;
>  typedef struct PropertyInfo PropertyInfo;
>  typedef struct Property Property;
>  typedef struct QEMUBH QEMUBH;
> diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> index a0e20b2..4f29055 100644
> --- a/migration/postcopy-ram.c
> +++ b/migration/postcopy-ram.c
> @@ -24,6 +24,7 @@
>  #include "migration/migration.h"
>  #include "migration/postcopy-ram.h"
>  #include "sysemu/sysemu.h"
> +#include "qemu/bitmap.h"
>  #include "qemu/error-report.h"
>  #include "trace.h"
>  
> @@ -49,6 +50,220 @@
>  
>  #if defined(__linux__) && defined(__NR_userfaultfd)
>  
> +/* ---------------------------------------------------------------------- */
> +/* Postcopy pagemap-inbound (pmi) - data structures that record the       */
> +/* state of each page used by the inbound postcopy                        */
> +/* It's a pair of bitmaps (of the same structure as the migration bitmaps)*/
> +/* holding one bit per target-page, although most operations work on host */
> +/* pages, the exception being a hook that receives incoming pages off the */
> +/* migration stream which come in a TP at a time, although the source     */
> +/* _should_ guarantee it sends a sequence of TPs representing HPs during  */
> +/* the postcopy phase, there is no such guarantee during precopy.  We     */
> +/* could boil this down to only holding one bit per-host page, but we lose*/
> +/* sanity checking that we really do get whole host-pages from the source.*/
> +__attribute__ (( unused )) /* Until later in patch series */
> +static void postcopy_pmi_init(MigrationIncomingState *mis, size_t ram_pages)
> +{
> +    unsigned int tpb = qemu_target_page_bits();
> +    unsigned long host_bits;
> +
> +    qemu_mutex_init(&mis->postcopy_pmi.mutex);
> +    mis->postcopy_pmi.state0 = bitmap_new(ram_pages);
> +    mis->postcopy_pmi.state1 = bitmap_new(ram_pages);
> +    bitmap_clear(mis->postcopy_pmi.state0, 0, ram_pages);
> +    bitmap_clear(mis->postcopy_pmi.state1, 0, ram_pages);
> +    /*
> +     * Each bit in the map represents one 'target page' which is no bigger
> +     * than a host page but can be smaller.  It's useful to have some
> +     * convenience masks for later
> +     */
> +
> +    /*
> +     * The number of bits one host page takes up in the bitmap
> +     * e.g. on a 64k host page, 4k Target page, host_bits=64/4=16
> +     */
> +    host_bits = getpagesize() / (1ul << tpb);

That's equivalent to getpagesize() >> tpb, isn't it?

> +    assert(is_power_of_2(host_bits));
> +
> +    mis->postcopy_pmi.host_bits = host_bits;
> +
> +    if (host_bits < BITS_PER_LONG) {
> +        /* A mask starting at bit 0 containing host_bits continuous set bits */
> +        mis->postcopy_pmi.host_mask =  (1ul << host_bits) - 1;
> +    } else {
> +        /*
> +         * This is a host where the ratio between host and target pages is
> +         * bigger than the size of our longs, so we can't make a mask
> +         * but we are only losing sanity checking if we just check one long's
> +         * worth of bits.
> +         */
> +        mis->postcopy_pmi.host_mask = ~0l;
> +    }
> +
> +
> +    assert((ram_pages % host_bits) == 0);
> +}
> +
> +void postcopy_pmi_destroy(MigrationIncomingState *mis)
> +{
> +    g_free(mis->postcopy_pmi.state0);
> +    mis->postcopy_pmi.state0 = NULL;
> +    g_free(mis->postcopy_pmi.state1);
> +    mis->postcopy_pmi.state1 = NULL;
> +    qemu_mutex_destroy(&mis->postcopy_pmi.mutex);
> +}
> +
> +/*
> + * Mark a set of pages in the PMI as being clear; this is used by the discard
> + * at the start of postcopy, and before the postcopy stream starts.
> + */
> +void postcopy_pmi_discard_range(MigrationIncomingState *mis,
> +                                size_t start, size_t npages)
> +{
> +    /* Clear to state 0 = missing */
> +    bitmap_clear(mis->postcopy_pmi.state0, start, npages);
> +    bitmap_clear(mis->postcopy_pmi.state1, start, npages);
> +}
> +
> +/*
> + * Test a host-page worth of bits in the map starting at bitmap_index
> + * The bits should all be consistent
> + */
> +static bool test_hpbits(MigrationIncomingState *mis,
> +                        size_t bitmap_index, unsigned long *map)
> +{
> +    long masked;
> +
> +    assert((bitmap_index & (mis->postcopy_pmi.host_bits-1)) == 0);
> +
> +    masked = (map[BIT_WORD(bitmap_index)] >>
> +               (bitmap_index % BITS_PER_LONG)) &
> +             mis->postcopy_pmi.host_mask;
> +
> +    assert((masked == 0) || (masked == mis->postcopy_pmi.host_mask));
> +    return !!masked;
> +}
> +
> +/*
> + * Set host-page worth of bits in the map starting at bitmap_index
> + * to the given state
> + */
> +static void set_hp(MigrationIncomingState *mis,
> +                   size_t bitmap_index, PostcopyPMIState state)
> +{
> +    long shifted_mask = mis->postcopy_pmi.host_mask <<
> +                        (bitmap_index % BITS_PER_LONG);
> +
> +    assert((bitmap_index & (mis->postcopy_pmi.host_bits-1)) == 0);

assert(state != 0)?

> +
> +    if (state & 1) {

Using the symbolic constants for PostcopyPMIState values here might be
better.

> +        mis->postcopy_pmi.state0[BIT_WORD(bitmap_index)] |= shifted_mask;
> +    } else {
> +        mis->postcopy_pmi.state0[BIT_WORD(bitmap_index)] &= ~shifted_mask;
> +    }
> +    if (state & 2) {
> +        mis->postcopy_pmi.state1[BIT_WORD(bitmap_index)] |= shifted_mask;
> +    } else {
> +        mis->postcopy_pmi.state1[BIT_WORD(bitmap_index)] &= ~shifted_mask;
> +    }
> +}
> +
> +/*
> + * Retrieve the state of the given page
> + * Note: This version for use by callers already holding the lock
> + */
> +static PostcopyPMIState postcopy_pmi_get_state_nolock(
> +                            MigrationIncomingState *mis,
> +                            size_t bitmap_index)
> +{
> +    bool b0, b1;
> +
> +    b0 = test_hpbits(mis, bitmap_index, mis->postcopy_pmi.state0);
> +    b1 = test_hpbits(mis, bitmap_index, mis->postcopy_pmi.state1);
> +
> +    return (b0 ? 1 : 0) + (b1 ? 2 : 0);

Ugh.. this is a hidden dependency on the PostcopyPMIState enum
elements never changing value.  Safer to code it as:
      if (!b0 && !b1) {
          return POSTCOPY_PMI_MISSING;
      } else if (...)
           ...

and let gcc sort it out.

> +}
> +
> +/* Retrieve the state of the given page */
> +__attribute__ (( unused )) /* Until later in patch series */
> +static PostcopyPMIState postcopy_pmi_get_state(MigrationIncomingState *mis,
> +                                               size_t bitmap_index)
> +{
> +    PostcopyPMIState ret;
> +    qemu_mutex_lock(&mis->postcopy_pmi.mutex);
> +    ret = postcopy_pmi_get_state_nolock(mis, bitmap_index);
> +    qemu_mutex_unlock(&mis->postcopy_pmi.mutex);
> +
> +    return ret;
> +}
> +
> +/*
> + * Set the page state to the given state if the previous state was as expected
> + * Return the actual previous state.
> + */
> +__attribute__ (( unused )) /* Until later in patch series */
> +static PostcopyPMIState postcopy_pmi_change_state(MigrationIncomingState *mis,
> +                                           size_t bitmap_index,
> +                                           PostcopyPMIState expected_state,
> +                                           PostcopyPMIState new_state)
> +{
> +    PostcopyPMIState old_state;
> +
> +    qemu_mutex_lock(&mis->postcopy_pmi.mutex);
> +    old_state = postcopy_pmi_get_state_nolock(mis, bitmap_index);
> +
> +    if (old_state == expected_state) {
> +        switch (new_state) {
> +        case POSTCOPY_PMI_MISSING:
> +            assert(0); /* This shouldn't happen - use discard_range */
> +            break;
> +
> +        case POSTCOPY_PMI_REQUESTED:
> +            assert(old_state == POSTCOPY_PMI_MISSING);
> +            /* missing -> requested */
> +            set_hp(mis, bitmap_index, POSTCOPY_PMI_REQUESTED);
> +            break;
> +
> +        case POSTCOPY_PMI_RECEIVED:
> +            assert(old_state == POSTCOPY_PMI_MISSING ||
> +                   old_state == POSTCOPY_PMI_REQUESTED);
> +            /* -> received */
> +            set_hp(mis, bitmap_index, POSTCOPY_PMI_RECEIVED);
> +            break;
> +        }
> +    }
> +
> +    qemu_mutex_unlock(&mis->postcopy_pmi.mutex);
> +    return old_state;
> +}
> +
> +/*
> + * Useful when debugging postcopy, although if it failed early the
> + * received map can be quite sparse and thus big when dumped.
> + */
> +void postcopy_pmi_dump(MigrationIncomingState *mis)
> +{
> +    fprintf(stderr, "postcopy_pmi_dump: bit 0\n");
> +    ram_debug_dump_bitmap(mis->postcopy_pmi.state0, false);
> +    fprintf(stderr, "postcopy_pmi_dump: bit 1\n");
> +    ram_debug_dump_bitmap(mis->postcopy_pmi.state1, true);
> +    fprintf(stderr, "postcopy_pmi_dump: end\n");
> +}
> +
> +/* Called by ram_load prior to mapping the page */
> +void postcopy_hook_early_receive(MigrationIncomingState *mis,
> +                                 size_t bitmap_index)
> +{
> +    if (mis->postcopy_state == POSTCOPY_INCOMING_ADVISE) {
> +        /*
> +         * If we're in precopy-advise mode we need to track received pages even
> +         * though we don't need to place pages atomically yet.
> +         * In advise mode there's only a single thread, so don't need locks
> +         */
> +        set_bit(bitmap_index, mis->postcopy_pmi.state1); /* 2=received */

Yeah.. so this bypasses postcopy_pmi_{get,change}_state, which again
makes me wonder whether the enum serves any purpose.

> +    }
> +}
> +
>  static bool ufd_version_check(int ufd)
>  {
>      struct uffdio_api api_struct;
> @@ -71,6 +286,7 @@ static bool ufd_version_check(int ufd)
>      return true;
>  }
>  
> +

Extraneous whitespace change.

>  bool postcopy_ram_supported_by_host(void)
>  {
>      long pagesize = getpagesize();
> @@ -157,5 +373,12 @@ bool postcopy_ram_supported_by_host(void)
>      return false;
>  }
>  
> +/* Called by ram_load prior to mapping the page */
> +void postcopy_hook_early_receive(MigrationIncomingState *mis,
> +                                 size_t bitmap_index)
> +{
> +    /* We don't support postcopy so don't care */
> +}
> +
>  #endif
>  

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

[-- Attachment #2: Type: application/pgp-signature, Size: 819 bytes --]