Re: Commit 282d8998e997 (srcu: Prevent expedited GPs and blocking readers from consuming CPU) cause qemu boot slow
From: Paul E. McKenney
Date: Mon Jun 13 2022 - 11:08:57 EST
On Mon, Jun 13, 2022 at 02:55:47PM +0800, zhangfei.gao@xxxxxxxxxxx wrote:
> Hi, Paul
>
> On 2022/6/13 下午12:16, Paul E. McKenney wrote:
> > On Sun, Jun 12, 2022 at 08:57:11PM -0700, Paul E. McKenney wrote:
> > > On Mon, Jun 13, 2022 at 11:04:39AM +0800, zhangfei.gao@xxxxxxxxxxx wrote:
> > > > Hi, Paul
> > > >
> > > > On 2022/6/13 上午2:49, Paul E. McKenney wrote:
> > > > > On Sun, Jun 12, 2022 at 07:29:30PM +0200, Paolo Bonzini wrote:
> > > > > > On 6/12/22 18:40, Paul E. McKenney wrote:
> > > > > > > > Do these reserved memory regions really need to be allocated separately?
> > > > > > > > (For example, are they really all non-contiguous? If not, that is, if
> > > > > > > > there are a lot of contiguous memory regions, could you sort the IORT
> > > > > > > > by address and do one ioctl() for each set of contiguous memory regions?)
> > > > > > > >
> > > > > > > > Are all of these reserved memory regions set up before init is spawned?
> > > > > > > >
> > > > > > > > Are all of these reserved memory regions set up while there is only a
> > > > > > > > single vCPU up and running?
> > > > > > > >
> > > > > > > > Is the SRCU grace period really needed in this case? (I freely confess
> > > > > > > > to not being all that familiar with KVM.)
> > > > > > > Oh, and there was a similar many-requests problem with networking many
> > > > > > > years ago. This was solved by adding a new syscall/ioctl()/whatever
> > > > > > > that permitted many requests to be presented to the kernel with a single
> > > > > > > system call.
> > > > > > >
> > > > > > > Could a new ioctl() be introduced that requested a large number
> > > > > > > of these memory regions in one go so as to make each call to
> > > > > > > synchronize_rcu_expedited() cover a useful fraction of your 9000+
> > > > > > > requests? Adding a few of the KVM guys on CC for their thoughts.
> > > > > > Unfortunately not. Apart from this specific case, in general the calls to
> > > > > > KVM_SET_USER_MEMORY_REGION are triggered by writes to I/O registers in the
> > > > > > guest, and those writes then map to a ioctl. Typically the guest sets up a
> > > > > > device at a time, and each setup step causes a synchronize_srcu()---and
> > > > > > expedited at that.
> > > > > I was afraid of something like that...
> > > > >
> > > > > > KVM has two SRCUs:
> > > > > >
> > > > > > 1) kvm->irq_srcu is hardly relying on the "sleepable" part; it has readers
> > > > > > that are very very small, but it needs extremely fast detection of grace
> > > > > > periods; see commit 719d93cd5f5c ("kvm/irqchip: Speed up
> > > > > > KVM_SET_GSI_ROUTING", 2014-05-05) which split it off kvm->srcu. Readers are
> > > > > > not so frequent.
> > > > > >
> > > > > > 2) kvm->srcu is nastier because there are readers all the time. The
> > > > > > read-side critical section are still short-ish, but they need the sleepable
> > > > > > part because they access user memory.
> > > > > Which one of these two is in play in this case?
> > > > >
> > > > > > Writers are not frequent per se; the problem is they come in very large
> > > > > > bursts when a guest boots. And while the whole boot path overall can be
> > > > > > quadratic, O(n) expensive calls to synchronize_srcu() can have a larger
> > > > > > impact on runtime than the O(n^2) parts, as demonstrated here.
> > > > > >
> > > > > > Therefore, we operated on the assumption that the callers of
> > > > > > synchronized_srcu_expedited were _anyway_ busy running CPU-bound guest code
> > > > > > and the desire was to get past the booting phase as fast as possible. If
> > > > > > the guest wants to eat host CPU it can "for(;;)" as much as it wants;
> > > > > > therefore, as long as expedited GPs didn't eat CPU *throughout the whole
> > > > > > system*, a preemptable busy wait in synchronize_srcu_expedited() were not
> > > > > > problematic.
> > > > > >
> > > > > > This assumptions did match the SRCU code when kvm->srcu and kvm->irq_srcu
> > > > > > were was introduced (respectively in 2009 and 2014). But perhaps they do
> > > > > > not hold anymore now that each SRCU is not as independent as it used to be
> > > > > > in those years, and instead they use workqueues instead?
> > > > > The problem was not internal to SRCU, but rather due to the fact
> > > > > that kernel live patching (KLP) had problems with the CPU-bound tasks
> > > > > resulting from repeated synchronize_rcu_expedited() invocations. So I
> > > > > added heuristics to get the occasional sleep in there for KLP's benefit.
> > > > > Perhaps these heuristics need to be less aggressive about adding sleep.
> > > > >
> > > > > These heuristics have these aspects:
> > > > >
> > > > > 1. The longer readers persist in an expedited SRCU grace period,
> > > > > the longer the wait between successive checks of the reader
> > > > > state. Roughly speaking, we wait as long as the grace period
> > > > > has currently been in effect, capped at ten jiffies.
> > > > >
> > > > > 2. SRCU grace periods have several phases. We reset so that each
> > > > > phase starts by not waiting (new phase, new set of readers,
> > > > > so don't penalize this set for the sins of the previous set).
> > > > > But once we get to the point of adding delay, we add the
> > > > > delay based on the beginning of the full grace period.
> > > > >
> > > > > Right now, the checking for grace-period length does not allow for the
> > > > > possibility that a grace period might start just before the jiffies
> > > > > counter gets incremented (because I didn't realize that anyone cared),
> > > > > so that is one possible thing to change. I can also allow more no-delay
> > > > > checks per SRCU grace-period phase.
> > > > >
> > > > > Zhangfei, does something like the patch shown below help?
> > > > >
> > > > > Additional adjustments are likely needed to avoid re-breaking KLP,
> > > > > but we have to start somewhere...
> > > > >
> > > > > Thanx, Paul
> > > > >
> > > > > ------------------------------------------------------------------------
> > > > >
> > > > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > > > > index 50ba70f019dea..6a354368ac1d1 100644
> > > > > --- a/kernel/rcu/srcutree.c
> > > > > +++ b/kernel/rcu/srcutree.c
> > > > > @@ -513,7 +513,7 @@ static bool srcu_readers_active(struct srcu_struct *ssp)
> > > > > #define SRCU_INTERVAL 1 // Base delay if no expedited GPs pending.
> > > > > #define SRCU_MAX_INTERVAL 10 // Maximum incremental delay from slow readers.
> > > > > -#define SRCU_MAX_NODELAY_PHASE 1 // Maximum per-GP-phase consecutive no-delay instances.
> > > > > +#define SRCU_MAX_NODELAY_PHASE 3 // Maximum per-GP-phase consecutive no-delay instances.
> > > > > #define SRCU_MAX_NODELAY 100 // Maximum consecutive no-delay instances.
> > > > > /*
> > > > > @@ -522,12 +522,18 @@ static bool srcu_readers_active(struct srcu_struct *ssp)
> > > > > */
> > > > > static unsigned long srcu_get_delay(struct srcu_struct *ssp)
> > > > > {
> > > > > + unsigned long gpstart;
> > > > > + unsigned long j;
> > > > > unsigned long jbase = SRCU_INTERVAL;
> > > > > if (ULONG_CMP_LT(READ_ONCE(ssp->srcu_gp_seq), READ_ONCE(ssp->srcu_gp_seq_needed_exp)))
> > > > > jbase = 0;
> > > > > - if (rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)))
> > > > > - jbase += jiffies - READ_ONCE(ssp->srcu_gp_start);
> > > > > + if (rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq))) {
> > > > > + j = jiffies - 1;
> > > > > + gpstart = READ_ONCE(ssp->srcu_gp_start);
> > > > > + if (time_after(j, gpstart))
> > > > > + jbase += j - gpstart;
> > > > > + }
> > > > > if (!jbase) {
> > > > > WRITE_ONCE(ssp->srcu_n_exp_nodelay, READ_ONCE(ssp->srcu_n_exp_nodelay) + 1);
> > > > > if (READ_ONCE(ssp->srcu_n_exp_nodelay) > SRCU_MAX_NODELAY_PHASE)
> > > > Unfortunately, this patch does not helpful.
> > > >
> > > > Then re-add the debug info.
> > > >
> > > > During the qemu boot
> > > > [ 232.997667] __synchronize_srcu loop=1000
> > > >
> > > > [ 361.094493] __synchronize_srcu loop=9000
> > > > [ 361.094501] Call trace:
> > > > [ 361.094502] dump_backtrace+0xe4/0xf0
> > > > [ 361.094505] show_stack+0x20/0x70
> > > > [ 361.094507] dump_stack_lvl+0x8c/0xb8
> > > > [ 361.094509] dump_stack+0x18/0x34
> > > > [ 361.094511] __synchronize_srcu+0x120/0x128
> > > > [ 361.094514] synchronize_srcu_expedited+0x2c/0x40
> > > > [ 361.094515] kvm_swap_active_memslots+0x130/0x198
> > > > [ 361.094519] kvm_activate_memslot+0x40/0x68
> > > > [ 361.094520] kvm_set_memslot+0x2f8/0x3b0
> > > > [ 361.094523] __kvm_set_memory_region+0x2e4/0x438
> > > > [ 361.094524] kvm_set_memory_region+0x78/0xb8
> > > > [ 361.094526] kvm_vm_ioctl+0x5a0/0x13e0
> > > > [ 361.094528] __arm64_sys_ioctl+0xb0/0xf8
> > > > [ 361.094530] invoke_syscall+0x4c/0x110
> > > > [ 361.094533] el0_svc_common.constprop.0+0x68/0x128
> > > > [ 361.094536] do_el0_svc+0x34/0xc0
> > > > [ 361.094538] el0_svc+0x30/0x98
> > > > [ 361.094541] el0t_64_sync_handler+0xb8/0xc0
> > > > [ 361.094544] el0t_64_sync+0x18c/0x190
> > > > [ 363.942817] kvm_set_memory_region loop=6000
> > > Huh.
> > >
> > > One possibility is that the "if (!jbase)" block needs to be nested
> > > within the "if (rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq))) {" block.
>
> I test this diff and NO helpful
>
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 50ba70f019de..36286a4b74e6 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -513,7 +513,7 @@ static bool srcu_readers_active(struct srcu_struct *ssp)
>
> #define SRCU_INTERVAL 1 // Base delay if no expedited GPs
> pending.
> #define SRCU_MAX_INTERVAL 10 // Maximum incremental delay from
> slow readers.
> -#define SRCU_MAX_NODELAY_PHASE 1 // Maximum per-GP-phase consecutive
> no-delay instances.
> +#define SRCU_MAX_NODELAY_PHASE 3 // Maximum per-GP-phase consecutive
> no-delay instances.
> #define SRCU_MAX_NODELAY 100 // Maximum consecutive no-delay
> instances.
>
> /*
> @@ -522,16 +522,23 @@ static bool srcu_readers_active(struct srcu_struct
> *ssp)
> */
> static unsigned long srcu_get_delay(struct srcu_struct *ssp)
> {
> + unsigned long gpstart;
> + unsigned long j;
> unsigned long jbase = SRCU_INTERVAL;
>
> if (ULONG_CMP_LT(READ_ONCE(ssp->srcu_gp_seq),
> READ_ONCE(ssp->srcu_gp_seq_needed_exp)))
> jbase = 0;
> - if (rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)))
> - jbase += jiffies - READ_ONCE(ssp->srcu_gp_start);
> - if (!jbase) {
> - WRITE_ONCE(ssp->srcu_n_exp_nodelay,
> READ_ONCE(ssp->srcu_n_exp_nodelay) + 1);
> - if (READ_ONCE(ssp->srcu_n_exp_nodelay) >
> SRCU_MAX_NODELAY_PHASE)
> - jbase = 1;
> + if (rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq))) {
> + j = jiffies - 1;
> + gpstart = READ_ONCE(ssp->srcu_gp_start);
> + if (time_after(j, gpstart))
> + jbase += j - gpstart;
> +
> + if (!jbase) {
> + WRITE_ONCE(ssp->srcu_n_exp_nodelay,
> READ_ONCE(ssp->srcu_n_exp_nodelay) + 1);
> + if (READ_ONCE(ssp->srcu_n_exp_nodelay) >
> SRCU_MAX_NODELAY_PHASE)
> + jbase = 1;
> + }
> }
That is in fact what I was intending you to test, thank you. As you
say, unfortunately it did not help.
Could you please test removing the "if (!jbase)" block entirely?
> > And when I run 10,000 consecutive synchronize_rcu_expedited() calls, the
> > above change reduces the overhead by more than an order of magnitude.
> > Except that the overhead of the series is far less than one second,
> > not the several minutes that you are seeing. So the per-call overhead
> > decreases from about 17 microseconds to a bit more than one microsecond.
> >
> > I could imagine an extra order of magnitude if you are running HZ=100
> > instead of the HZ=1000 that I am running. But that only gets up to a
> > few seconds.
One possible reason for the difference would be if your code has
SRCU readers.
Could you please tell me the value of CONFIG_HZ on your system?
Also the value of CONFIG_PREEMPTION?
> > > One additional debug is to apply the patch below on top of the one you
> apply the patch below?
> > > just now kindly tested, then use whatever debug technique you wish to
> > > work out what fraction of the time during that critical interval that
> > > srcu_get_delay() returns non-zero.
> Sorry, I am confused, no patch right?
Apologies, my omission.
> Just measure srcu_get_delay return to non-zero?
Exactly, please!
> By the way, the issue should be only related with qemu apci. not related
> with rmr feature
> Test with: https://github.com/qemu/qemu/tree/stable-6.1
>
> Looks it caused by too many kvm_region_add & kvm_region_del if acpi=force,
> If no acpi, no print kvm_region_add/del (1000 times print once)
>
> If with acpi=force,
> During qemu boot
> kvm_region_add region_add = 1000
> kvm_region_del region_del = 1000
> kvm_region_add region_add = 2000
> kvm_region_del region_del = 2000
> kvm_region_add region_add = 3000
> kvm_region_del region_del = 3000
> kvm_region_add region_add = 4000
> kvm_region_del region_del = 4000
> kvm_region_add region_add = 5000
> kvm_region_del region_del = 5000
> kvm_region_add region_add = 6000
> kvm_region_del region_del = 6000
>
> kvm_region_add/kvm_region_del ->
> kvm_set_phys_mem->
> kvm_set_user_memory_region->
> kvm_vm_ioctl(s, KVM_SET_USER_MEMORY_REGION, &mem)
>
> [ 361.094493] __synchronize_srcu loop=9000
> [ 361.094501] Call trace:
> [ 361.094502] dump_backtrace+0xe4/0xf0
> [ 361.094505] show_stack+0x20/0x70
> [ 361.094507] dump_stack_lvl+0x8c/0xb8
> [ 361.094509] dump_stack+0x18/0x34
> [ 361.094511] __synchronize_srcu+0x120/0x128
> [ 361.094514] synchronize_srcu_expedited+0x2c/0x40
> [ 361.094515] kvm_swap_active_memslots+0x130/0x198
> [ 361.094519] kvm_activate_memslot+0x40/0x68
> [ 361.094520] kvm_set_memslot+0x2f8/0x3b0
> [ 361.094523] __kvm_set_memory_region+0x2e4/0x438
> [ 361.094524] kvm_set_memory_region+0x78/0xb8
> [ 361.094526] kvm_vm_ioctl+0x5a0/0x13e0
> [ 361.094528] __arm64_sys_ioctl+0xb0/0xf8
> [ 361.094530] invoke_syscall+0x4c/0x110
> [ 361.094533] el0_svc_common.constprop.0+0x68/0x128
> [ 361.094536] do_el0_svc+0x34/0xc0
> [ 361.094538] el0_svc+0x30/0x98
> [ 361.094541] el0t_64_sync_handler+0xb8/0xc0
> [ 361.094544] el0t_64_sync+0x18c/0x190
> [ 363.942817] kvm_set_memory_region loop=6000
Good to know, thank you!
Thanx, Paul