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authorThomas Gleixner <tglx@linutronix.de>2019-11-12 01:27:09 +0000
committerThomas Gleixner <tglx@linutronix.de>2020-01-14 12:20:57 +0100
commit660fd04f9317172ae90f414c68b18a26ae88a829 (patch)
tree753b860018b0f2a5dbad08a91c684ab8d6795d66 /init
parent6f74acfde20af1eb2178d0bd846bfd8f50b3be32 (diff)
downloadlwn-660fd04f9317172ae90f414c68b18a26ae88a829.tar.gz
lwn-660fd04f9317172ae90f414c68b18a26ae88a829.zip
lib/vdso: Prepare for time namespace support
To support time namespaces in the vdso with a minimal impact on regular non time namespace affected tasks, the namespace handling needs to be hidden in a slow path. The most obvious place is vdso_seq_begin(). If a task belongs to a time namespace then the VVAR page which contains the system wide vdso data is replaced with a namespace specific page which has the same layout as the VVAR page. That page has vdso_data->seq set to 1 to enforce the slow path and vdso_data->clock_mode set to VCLOCK_TIMENS to enforce the time namespace handling path. The extra check in the case that vdso_data->seq is odd, e.g. a concurrent update of the vdso data is in progress, is not really affecting regular tasks which are not part of a time namespace as the task is spin waiting for the update to finish and vdso_data->seq to become even again. If a time namespace task hits that code path, it invokes the corresponding time getter function which retrieves the real VVAR page, reads host time and then adds the offset for the requested clock which is stored in the special VVAR page. If VDSO time namespace support is disabled the whole magic is compiled out. Initial testing shows that the disabled case is almost identical to the host case which does not take the slow timens path. With the special timens page installed the performance hit is constant time and in the range of 5-7%. For the vdso functions which are not using the sequence count an unconditional check for vdso_data->clock_mode is added which switches to the real vdso when the clock_mode is VCLOCK_TIMENS. [avagin: Make do_hres_timens() work with raw clocks too: choose vdso_data pointer by CS_RAW offset.] Suggested-by: Andy Lutomirski <luto@kernel.org> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Andrei Vagin <avagin@gmail.com> Signed-off-by: Dmitry Safonov <dima@arista.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Link: https://lore.kernel.org/r/20191112012724.250792-21-dima@arista.com
Diffstat (limited to 'init')
-rw-r--r--init/Kconfig1
1 files changed, 1 insertions, 0 deletions
diff --git a/init/Kconfig b/init/Kconfig
index b34314fc75f7..9b7f144a6d35 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1082,6 +1082,7 @@ config UTS_NS
config TIME_NS
bool "TIME namespace"
+ depends on GENERIC_VDSO_TIME_NS
default y
help
In this namespace boottime and monotonic clocks can be set.