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Re: [Qemu-devel] [PATCH v5 26/45] Postcopy page-map-incoming (PMI) struc
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] [PATCH v5 26/45] Postcopy page-map-incoming (PMI) structure |
Date: |
Fri, 13 Mar 2015 13:47:53 +0000 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
* David Gibson (address@hidden) wrote:
> On Wed, Feb 25, 2015 at 04:51:49PM +0000, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <address@hidden>
> >
> > 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 <address@hidden>
> > ---
> > 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.
Thanks; (wth didn't scripts/checkpatch spot that?)
> > + 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.
Hmm; so most of the comments here come down to how this pair are represented.
I'd previously had a 'received' and 'requested' array pair, and only used
!received, !requested
received, !requested
!received, requested
but one of the intermediate changes (that never survived) I needed a 4th state,
and well I did have the 4th state it ended up encoded as received && requested;
but that wasn't actually what my 4th state meant, and so I thought it best
to try and get away from each of the bits meaning something and
try and move more towards just treating it as a state with the encoding
to the two state bits done in as few places as possible.
Really what I want is a nice dense efficient array of PostcopyPMIState's.
> > + 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"?
Done.
> > +};
> > +
> > 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?
Yes, fixed.
> > + 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)?
It could do, although again I'm just trying to make this encode things.
> > +
> > + if (state & 1) {
>
> Using the symbolic constants for PostcopyPMIState values here might be
> better.
I was treating this as the thing that encoded/decoded the enum; it
doesn't need to know the meanings of the bits.
>
> > + 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.
Again, I was trying to make this just the interface; so it doesn't
know or care about the enum mapping; we can change the enum mapping to
the bits without changing this function (or the callers) at all.
> > +}
> > +
> > +/* 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.
Yes, let me see how to fix that; this is the one place that deals in things
other than host-pages.
> > + }
> > +}
> > +
> > 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.
Oops; gone.
>
> > 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
> >
Thanks,
Dave
>
> --
> 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
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK