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Re: [Qemu-devel] [RFC 12/29] migration: allow dst vm pause on postcopy
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From: |
Dr. David Alan Gilbert |
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Subject: |
Re: [Qemu-devel] [RFC 12/29] migration: allow dst vm pause on postcopy |
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Date: |
Fri, 4 Aug 2017 10:33:19 +0100 |
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User-agent: |
Mutt/1.8.3 (2017-05-23) |
* Peter Xu (address@hidden) wrote:
> On Thu, Aug 03, 2017 at 03:03:57PM +0100, Dr. David Alan Gilbert wrote:
> > * Peter Xu (address@hidden) wrote:
> > > On Tue, Aug 01, 2017 at 10:47:16AM +0100, Dr. David Alan Gilbert wrote:
> > > > * Peter Xu (address@hidden) wrote:
> > >
> > > [...]
> > >
> > > > > +/* Return true if we should continue the migration, or false. */
> > > > > +static bool postcopy_pause_incoming(MigrationIncomingState *mis)
> > > > > +{
> > > > > + trace_postcopy_pause_incoming();
> > > > > +
> > > > > + migrate_set_state(&mis->state, MIGRATION_STATUS_POSTCOPY_ACTIVE,
> > > > > + MIGRATION_STATUS_POSTCOPY_PAUSED);
> > > > > +
> > > > > + assert(mis->from_src_file);
> > > > > + qemu_file_shutdown(mis->from_src_file);
> > > > > + qemu_fclose(mis->from_src_file);
> > > > > + mis->from_src_file = NULL;
> > > > > +
> > > > > + assert(mis->to_src_file);
> > > > > + qemu_mutex_lock(&mis->rp_mutex);
> > > > > + qemu_file_shutdown(mis->to_src_file);
> > > > > + qemu_fclose(mis->to_src_file);
> > > > > + mis->to_src_file = NULL;
> > > > > + qemu_mutex_unlock(&mis->rp_mutex);
> > > >
> > > > Hmm is that safe? If we look at migrate_send_rp_message we have:
> > > >
> > > > static void migrate_send_rp_message(MigrationIncomingState *mis,
> > > > enum mig_rp_message_type
> > > > message_type,
> > > > uint16_t len, void *data)
> > > > {
> > > > trace_migrate_send_rp_message((int)message_type, len);
> > > > qemu_mutex_lock(&mis->rp_mutex);
> > > > qemu_put_be16(mis->to_src_file, (unsigned int)message_type);
> > > > qemu_put_be16(mis->to_src_file, len);
> > > > qemu_put_buffer(mis->to_src_file, data, len);
> > > > qemu_fflush(mis->to_src_file);
> > > > qemu_mutex_unlock(&mis->rp_mutex);
> > > > }
> > > >
> > > > If we came into postcopy_pause_incoming at about the same time
> > > > migrate_send_rp_message was being called and pause_incoming took the
> > > > lock first, then once it release the lock, send_rp_message carries on
> > > > and uses mis->to_src_file that's now NULL.
> > > >
> > > > One solution here is to just call qemu_file_shutdown() but leave the
> > > > files open at this point, but clean the files up sometime later.
> > >
> > > I see the commnent on patch 14 as well - yeah, we need patch 14 to
> > > co-op here, and as long as we are with patch 14, we should be ok.
> > >
> > > >
> > > > > +
> > > > > + while (mis->state == MIGRATION_STATUS_POSTCOPY_PAUSED) {
> > > > > + qemu_sem_wait(&mis->postcopy_pause_sem_dst);
> > > > > + }
> > > > > +
> > > > > + trace_postcopy_pause_incoming_continued();
> > > > > +
> > > > > + return true;
> > > > > +}
> > > > > +
> > > > > static int qemu_loadvm_state_main(QEMUFile *f,
> > > > > MigrationIncomingState *mis)
> > > > > {
> > > > > uint8_t section_type;
> > > > > int ret = 0;
> > > > >
> > > > > +retry:
> > > > > while (true) {
> > > > > section_type = qemu_get_byte(f);
> > > > >
> > > > > @@ -2004,6 +2034,21 @@ static int qemu_loadvm_state_main(QEMUFile *f,
> > > > > MigrationIncomingState *mis)
> > > > > out:
> > > > > if (ret < 0) {
> > > > > qemu_file_set_error(f, ret);
> > > > > +
> > > > > + /*
> > > > > + * Detect whether it is:
> > > > > + *
> > > > > + * 1. postcopy running
> > > > > + * 2. network failure (-EIO)
> > > > > + *
> > > > > + * If so, we try to wait for a recovery.
> > > > > + */
> > > > > + if (mis->state == MIGRATION_STATUS_POSTCOPY_ACTIVE &&
> > > > > + ret == -EIO && postcopy_pause_incoming(mis)) {
> > > > > + /* Reset f to point to the newly created channel */
> > > > > + f = mis->from_src_file;
> > > > > + goto retry;
> > > > > + }
> > > >
> > > > I wonder if:
> > > >
> > > > if (mis->state == MIGRATION_STATUS_POSTCOPY_ACTIVE &&
> > > > ret == -EIO && postcopy_pause_incoming(mis)) {
> > > > /* Try again after postcopy recovery */
> > > > return qemu_loadvm_state_main(mis->from_src_file, mis);
> > > > }
> > > > would be nicer; it avoids the goto loop.
> > >
> > > I agree we should avoid using goto loops. However I do see vast usages
> > > of goto like this one when we want to redo part of the procedures of a
> > > function (or, of course, when handling errors in "C-style").
> >
> > We mostly use them to jump forward to an error exit; only rarely do
> > we do loops with them; so if we can sensibly avoid them it's best.
> >
> > > Calling qemu_loadvm_state_main() inside itself is ok as well, but it
> > > also has defect: stack usage would be out of control, or even, it can
> > > be controled by malicious users. E.g., if someone used program to
> > > periodically stop/start any network endpoint along the migration
> > > network, QEMU may go into a paused -> recovery -> active -> paused ...
> > > loop, and stack usage will just grow with time. I'd say it's an
> > > extreme example though...
> >
> > I think it's safe because it's a tail-call so a new stack frame isn't
> > needed.
>
> I tried it and dumped the assembly, looks like even with tail-call, we
> didn't really avoid the "callq":
>
> (gdb) disassemble qemu_loadvm_state_main
> Dump of assembler code for function qemu_loadvm_state_main:
> 0x00000000005d9ff8 <+0>: push %rbp
> 0x00000000005d9ff9 <+1>: mov %rsp,%rbp
> 0x00000000005d9ffc <+4>: sub $0x20,%rsp
> 0x00000000005da000 <+8>: mov %rdi,-0x18(%rbp)
> 0x00000000005da004 <+12>: mov %rsi,-0x20(%rbp)
> 0x00000000005da008 <+16>: movl $0x0,-0x4(%rbp)
> 0x00000000005da00f <+23>: mov -0x18(%rbp),%rax
> 0x00000000005da013 <+27>: mov %rax,%rdi
> 0x00000000005da016 <+30>: callq 0x5e185e <qemu_get_byte>
>
> [...]
>
> 0x00000000005da135 <+317>: jne 0x5da165 <qemu_loadvm_state_main+365>
> 0x00000000005da137 <+319>: cmpl $0xfffffffb,-0x4(%rbp)
> 0x00000000005da13b <+323>: jne 0x5da165 <qemu_loadvm_state_main+365>
> 0x00000000005da13d <+325>: mov -0x20(%rbp),%rax
> 0x00000000005da141 <+329>: mov %rax,%rdi
> 0x00000000005da144 <+332>: callq 0x5d9eb4 <postcopy_pause_incoming>
> 0x00000000005da149 <+337>: test %al,%al
> 0x00000000005da14b <+339>: je 0x5da165 <qemu_loadvm_state_main+365>
> 0x00000000005da14d <+341>: mov -0x20(%rbp),%rax
> 0x00000000005da151 <+345>: mov (%rax),%rax
> 0x00000000005da154 <+348>: mov -0x20(%rbp),%rdx
> 0x00000000005da158 <+352>: mov %rdx,%rsi
> 0x00000000005da15b <+355>: mov %rax,%rdi
> 0x00000000005da15e <+358>: callq 0x5d9ff8 <qemu_loadvm_state_main>
> ^^^^^^^^^^^^^^^ (this one)
> 0x00000000005da163 <+363>: jmp 0x5da168 <qemu_loadvm_state_main+368>
> 0x00000000005da165 <+365>: mov -0x4(%rbp),%eax
> 0x00000000005da168 <+368>: leaveq
> 0x00000000005da169 <+369>: retq
>
> Do we need extra compilation parameters to achieve the tail-call
> optimization for gcc? My gcc version is: v6.1.1 20160621.
>
> (even with extra flags, I am still a bit worried on whether it'll work
> on the other compilers though)
Huh, I'd expected it to be smarter than that; not sure why it didn't!
Anyway, tbh I wouldn't worry about the stack depth in this case.
> And, the "label-way" to retry is indeed used widely at least in both
> QEMU and Linux kernel. I tried to directly grep "^retry:" (so we are
> ignoring the same usage using different label names), there are ~30
> usage in QEMU and hundreds of cases in Linux kernel. So not sure
> whether this can be seen as another "legal" way to use C labels...
OK, my distaste for Goto's is perhaps a bit stronger than others;
it's OK though.
Dave
> Thanks,
>
> --
> Peter Xu
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK