From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Roger Pau Monne <roger.pau@citrix.com>, <xen-devel@lists.xenproject.org>
Cc: Wei Liu <wl@xen.org>, Jan Beulich <jbeulich@suse.com>
Subject: Re: [Xen-devel] [PATCH] x86/flush: use APIC ALLBUT destination shorthand when possible
Date: Fri, 27 Dec 2019 15:03:57 +0000 [thread overview]
Message-ID: <7e6ca9ea-89d4-80a6-d1e5-622ea87bf8a9@citrix.com> (raw)
In-Reply-To: <20191224124453.47183-1-roger.pau@citrix.com>
On 24/12/2019 12:44, Roger Pau Monne wrote:
> If the flush mask matches the mask of online CPUs use the APIC ALLBUT
> destination shorthand in order to send an IPI to all CPUs on the
> system except the current one. This can only be safely used when no
> CPU hotplug or unplug operations are taking place, no offline CPUs or
> those have been onlined and parked and finally when all CPUs in the
> system have been accounted for (ie: the number of CPUs doesn't exceed
> NR_CPUS and APIC IDs are below MAX_APICS).
>
> This is specially beneficial when using the PV shim, since using the
> shorthand avoids performing an APIC register write (or multiple ones
> if using xAPIC mode) for each destination in the flush mask.
>
> The lock time on a 32 vCPU guest using the shim without the shorthand
> is:
>
> Global lock flush_lock: addr=ffff82d0804b21c0, lockval=f602f602, not locked
> lock:228455938(79406065573135), block:205908580(556416605761539)
>
> Average lock time: 347577ns
>
> While the same guest using the shorthand:
>
> Global lock flush_lock: addr=ffff82d0804b41c0, lockval=d9c4d9bc, cpu=12
> lock:1890775(416719148054), block:1663958(2500161282949)
>
> Average lock time: 220395ns
>
> Approximately a 1/3 improvement in the lock time.
>
> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
While these are good stats, I'm somewhat hesitant about hacking this in
like this. For one, it is a substantial amount of ad-hoc logic in
flush_area_mask()
Shorthand safety really should be part of the apic driver, not part of
the TLB logic.
~Andrew
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next prev parent reply other threads:[~2019-12-27 15:04 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-24 12:44 [Xen-devel] [PATCH] x86/flush: use APIC ALLBUT destination shorthand when possible Roger Pau Monne
2019-12-27 15:03 ` Andrew Cooper [this message]
2019-12-31 12:13 ` Roger Pau Monné
2020-01-03 12:08 ` Jan Beulich
2020-01-03 12:24 ` Andrew Cooper
2020-01-03 12:34 ` Roger Pau Monné
2020-01-03 12:55 ` Jan Beulich
2020-01-08 13:30 ` Roger Pau Monné
2020-01-08 13:54 ` Jan Beulich
2020-01-08 18:14 ` Roger Pau Monné
2020-01-09 9:43 ` Jan Beulich
2020-01-09 11:24 ` Roger Pau Monné
2020-01-09 12:20 ` Roger Pau Monné
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