Re: [RFC PATCH 6/7] migration/multifd: Move payload storage out of the channel parameters

[Peter Xu <peterx@redhat.com>]

On Thu, Jun 27, 2024 at 10:40:11AM -0400, Peter Xu wrote:
> On Thu, Jun 27, 2024 at 11:27:08AM +0800, Wang, Lei wrote:
> > > Or graphically:
> > > 
> > > 1) client fills the active slot with data. Channels point to nothing
> > >    at this point:
> > >   [a]      <-- active slot
> > >   [][][][] <-- free slots, one per-channel
> > > 
> > >   [][][][] <-- channels' p->data pointers
> > > 
> > > 2) multifd_send() swaps the pointers inside the client slot. Channels
> > >    still point to nothing:
> > >   []
> > >   [a][][][]
> > > 
> > >   [][][][]
> > > 
> > > 3) multifd_send() finds an idle channel and updates its pointer:
> > 
> > It seems the action "finds an idle channel" is in step 2 rather than step 3,
> > which means the free slot is selected based on the id of the channel found, am I
> > understanding correctly?
> 
> I think you're right.
> 
> Actually I also feel like the desription here is ambiguous, even though I
> think I get what Fabiano wanted to say.
> 
> The free slot should be the first step of step 2+3, here what Fabiano
> really wanted to suggest is we move the free buffer array from multifd
> channels into the callers, then the caller can pass in whatever data to
> send.
> 
> So I think maybe it's cleaner to write it as this in code (note: I didn't
> really change the code, just some ordering and comments):
> 
> ===8<===
> @@ -710,15 +710,11 @@ static bool multifd_send(MultiFDSlots *slots)
>       */
>      active_slot = slots->active;
>      slots->active = slots->free[p->id];
> -    p->data = active_slot;
> -
> -    /*
> -     * By the next time we arrive here, the channel will certainly
> -     * have consumed the active slot. Put it back on the free list
> -     * now.
> -     */
>      slots->free[p->id] = active_slot;
>  
> +    /* Assign the current active slot to the chosen thread */
> +    p->data = active_slot;
> ===8<===
> 
> The comment I removed is slightly misleading to me too, because right now 
> active_slot contains the data hasn't yet been delivered to multifd, so
> we're "putting it back to free list" not because of it's free, but because
> we know it won't get used until the multifd send thread consumes it
> (because before that the thread will be busy, and we won't use the buffer
> if so in upcoming send()s).
> 
> And then when I'm looking at this again, I think maybe it's a slight
> overkill, and maybe we can still keep the "opaque data" managed by multifd.
> One reason might be that I don't expect the "opaque data" payload keep
> growing at all: it should really be either RAM or device state as I
> commented elsewhere in a relevant thread, after all it's a thread model
> only for migration purpose to move vmstates..
> 
> Putting it managed by multifd thread should involve less change than this
> series, but it could look like this:
> 
> typedef enum {
>     MULTIFD_PAYLOAD_RAM = 0,
>     MULTIFD_PAYLOAD_DEVICE_STATE = 1,
> } MultifdPayloadType;
> 
> typedef enum {
>     MultiFDPages_t ram_payload;
>     MultifdDeviceState_t device_payload;
> } MultifdPayload;

PS: please conditionally read "enum" as "union" throughout the previous
email of mine, sorry.

[I'll leave that to readers to decide when should do the replacement..]

> 
> struct MultiFDSendData {
>     MultifdPayloadType type;
>     MultifdPayload data;
> };
> 
> Then the "enum" makes sure the payload only consumes only the max of both
> types; a side benefit to save some memory.
> 
> I think we need to make sure MultifdDeviceState_t is generic enough so that
> it will work for mostly everything (especially normal VMSDs).  In this case
> the VFIO series should be good as that was currently defined as:
> 
> typedef struct {
>     MultiFDPacketHdr_t hdr;
> 
>     char idstr[256] QEMU_NONSTRING;
>     uint32_t instance_id;
> 
>     /* size of the next packet that contains the actual data */
>     uint32_t next_packet_size;
> } __attribute__((packed)) MultiFDPacketDeviceState_t;
> 
> IIUC that was what we need exactly with idstr+instance_id, so as to nail
> exactly at where should the "opaque device state" go to, then load it with
> a buffer-based loader when it's ready (starting from VFIO, to get rid of
> qemufile).  For VMSDs in the future if ever possible, that should be a
> modified version of vmstate_load() where it may take buffers not qemufiles.
> 
> To Maciej: please see whether above makes sense to you, and if you also
> agree please consider that with your VFIO work.
> 
> Thanks,
> 
> > 
> > >   []
> > >   [a][][][]
> > > 
> > >   [a][][][]
> > >   ^idle
> 
> -- 
> Peter Xu

-- 
Peter Xu