From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <44A3ACA8.6070201@domain.hid> Date: Thu, 29 Jun 2006 12:34:16 +0200 From: Jan Kiszka MIME-Version: 1.0 Subject: Re: [Xenomai-core] Prio-inversion on cleanup? References: <44A1608B.3090605@domain.hid> <44A295ED.2080306@domain.hid> <44A39C3C.1080508@domain.hid> <1151576082.5291.41.camel@domain.hid> In-Reply-To: <1151576082.5291.41.camel@domain.hid> Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="------------enig7494086C8D77FE13690ED61C" Sender: jan.kiszka@domain.hid List-Id: "Xenomai life and development \(bug reports, patches, discussions\)" List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: rpm@xenomai.org Cc: xenomai-core This is an OpenPGP/MIME signed message (RFC 2440 and 3156) --------------enig7494086C8D77FE13690ED61C Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Philippe Gerum wrote: > On Thu, 2006-06-29 at 11:24 +0200, Jan Kiszka wrote: >> Jan Kiszka wrote: >>> ... >>> The pthread is blocked on the irqbench device ioctl. On hitting ^C, >>> close() is invoked from the main thread for that device. The pthread = is >>> woken up and obviously relaxed on some linux syscall (after being >>> interrupted twice by the periodic timer event of a "latency -p 300 -P= >>> 50" instance). This passes the control over to the main thread while >>> keeping the pthread prio of 99. And this prio seems to survive for th= e >>> following 11 ms (full trace available on request). >>> >>> Any ideas what's going on? >>> >> Ok, I think I finally understood the issue. It seems to lie deep in th= e >> POSIX user-space lib, specifically its use of standard pthread service= s. >> Let me first clarify my scenario: >> >> A high-prio pthread of known and (theoretically) bounded workload shal= l >> be started and stopped while a low-prio thread is already running. The= >> low-prio thread shall only be impacted by the real workload of the >> high-prio one, not by its creation/destruction - at least not >> significantly. To achieve this with the POSIX skin (actually this >> applies to preempt-rt in theory as well), I have to create the thread >> under SCHED_OTHER, raise its priority right before entering the >> workload, and lower it again before leaving the thread. >> >> But, unfortunately, __wrap_pthread_setschedparam() depends on some >> real_pthread functions to be called. One of them is >> __real_pthread_setschedparam, and this one issues a linux syscall for >> obvious reasons. When lowering the thread to SCHED_OTHER, this syscall= >> is still issued under the original priority. And here we get bitten by= >> the prio-inheritance feature of the nucleus which, in my case, lets >> significant parts of standard Linux execute under high priority, >> delaying my other real-time threads. >> >> Now I wonder how to resolve this, how to make pthread_setschedparam (a= >> rather central RT-service) really real-time safe? I would say we need >> something like a lazy schedparam propagation to Linux which only takes= >> place when the thread enters secondary mode intentionally or no other = RT >> thread is ready. But I do not have a design for this at hand. Nasty. >> >> [My preferred way for every setup !=3D CONFIG_PREEMPT_RT + CONFIG_XENO= MAI >> would still be to switch this prio-inheritance off for the root thread= =2E >> But this was nack'ed by Philippe several times before... ;)] >> >=20 > I nacked the proposal to _always_ switch it off. Some applications > deeply need this. I think to remember asking for a CONFIG switch here. Some applications actually benefit while others (I even think most) do not need it or even easily screw themselves up during init/cleanup. You know, my old concerns. :) >=20 >> Side note: the native skin does not seem to suffer from this effect as= >> it only tracks the current prio at Xenomai level. >> >=20 > Switching off priority adjustment for the root thread before moving a > SCHED_FIFO shadow to SCHED_OTHER would prevent this side-effect. We'd > need to add a per-thread status bit to check whether we should run > xnpod_renice_root() or not for any given thread, and switch it on/off > from __wrap_pthread_setschedparam. >=20 This doesn't sound bad and would probably help low-prio threads also in some other scenarios. Nevertheless, a syscall-less pthread_setschedparam would still be a good idea as well, this time having the caller in mind who wishes to change its priority without entering Linux. Jan --------------enig7494086C8D77FE13690ED61C Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.2 (GNU/Linux) Comment: Using GnuPG with SUSE - http://enigmail.mozdev.org iD8DBQFEo6yoniDOoMHTA+kRAivcAJ0TMI6m5nkUNFzulBqKusuttRAS6gCeMA4B 8dh4r/h+ILuVHWeyFgmCwNI= =/xig -----END PGP SIGNATURE----- --------------enig7494086C8D77FE13690ED61C--